You may wish to look at inlays. Instead of alloying the materials, merely have a pocket cut out where you put the spectral metals.

In the real world, we have variable tempered blades. These are blades which are tempered such that the edge of the blade is very hard (and brittle), while the body of the sword is more flexible. This allows a blade to flex while still holding a very sharp edge.

However, you specifically mention that these spectral metals can take less stress than their mundane equivalents in every way. In the above example of variable tempers, the edge is hard, capable of taking great stress before chipping away. If the edge crumbles at lower stresses, it will never be an effective sword. The first time you use the blade, you'll end up damaging it.

So as an alternative, consider inlaying a small section of the edge with the spectral metals of choice. When fighting normally, you simply avoid using that particular section of the blade. When you need its particular behaviors, then you use it.

Also consider that some of your spectral metals don't even need to be on the edge. While the edge does the cutting, the sides of the blade do touch the target. If you attacked a faerie with a sword which was all steel except for a thin ridge 1/4" from the edge made of inlaid spectral iron, you'd certainly set them on fire, but you wouldn't have to sacrifice the hardness of your edge to do so. Spectral copper might be inlaid in circles near the back side of the blade, generating electric charges from there which flow through the steel.

Pattern welding

You start with three (or more) rods of different metals. You lay them side by side, with the softest in the middle and the hardest, most brittle on the outside; then heat them, fold them into a spiral and beat them flat. Heat, fold, and beat again; repeat until the metals blend together.

This method was used by the Vikings (among others) to make swords with a hard, sharp (but brittle) edge and a soft, elastic core, with a gradual transition from hard to soft.

Here is a tutorial about how to make a pattern welded sword.

It's actually remarkably simple to join two pieces of metal together for these purposes.

You place the metal parts together and hit them with a hammer until they join.

You're joking, it can't be that simple!

These days we have fancy names like cold pressure welding, and use machines that do it more efficiently, but in practice it's what blacksmiths have been doing for centuries. You can heat it up first which makes it easier, certainly since you're dealing with iron you'll want to heat it first for forge welding, but then you hit it with a hammer until it welds.

If you're wondering about availability of forge welding, any blacksmith would be able to do it. It's a fundamental aspect of their craft.

The solution would appear to be Electrolysis.

Electrolysis is a common process for plating metal objects with a more valuable metal. For example, Electroplating is a common way of putting gold, silver or nickel over the top of another metal. More recently, there appears to be a process which allows for this to be done with Iron.

If you use this process on your sword, there are two distinct advantages;

1) The bulk of your sword gets the benefit of mundane strength without reducing the efficacy of spectral contact on the blade

2) As the plate metal wears, you can always add more through a reapplication of the electroplating process.

You literally get the best of both worlds; you get a sword with all the strength of conventional metal but you also get all the spectral benefits that contact with the blade would imbue. And, you can always freshen up the blade between battles.

Differential Spectrality

In bladesmithing, there is a technique called differential hardening. This simply means that different parts of the blade are cooled at different rates. This creates different types of steel with different properties.

The Japanese achieved this by coating parts of their katana blades with clay before tempering them. This meant that the edge cooled slower, making it harder and more suitable for holding an edge, where as the back cooled quicker and was softer, meaning if the edge chipped (as the edge is very hard and prone to damage), a crack wouldn’t form across the whole blade and cause it to break in two. (As a side note, this is why the katana curves, the back cooling faster than the edge means the steel is pulled back. If you wanted your blade to be straight, you would make it curved forwards before tempering so, when it bent back, it would be straight.)

Applying this to your question, your swords may undergo ‘differential spectrality’ where the edge of the blade is made to be more spectral than the back of the blade. The entire blade might initially be made of the same metal, such as steel, but you coat the back of the blade in, lets say, clay. This mean when you make the metal spectral, only the edge is spectral, the back of it is still very mundane as the clay prevented the spectral energy infusing the metal underneath it. Once it has finished undergoing the process, you would have a blade with a mundane steel back with a spectral steel edge.

One technique you could use is a bastardization of the San Mai technique.

Simply, San Mai is a technique that builds off forge welding - heat multiple pieces of metal up and apply pressure (hammer) until they become one solid piece. This is how one makes damascus or pattern-welded steel - Forge weld several layers of different steels together, cut in in half or fold it or twist it (whatever) and keep doing that a few times to get a really interesting pattern.

San Mai is a variation on that that's more simple to grasp - you effectively take one steel, fold it into a taco shape, and inset another steel as the "meat" of the taco. Then proceed to forge weld those two pieces together (but you don't add extra steps - no folding, twisting, etc). Typically that'd be used to have a hard steel edge and core, with a much softer steel case to better absorb the shocks (the softer steel is literally less harden-able, so you don't have to worry about differential heating as much, just get the whole piece to heat and quench the whole thing).

What I'd imagine as your "masterwork" case would be some variation on this. Where you first make a billet that has the spectral properties you want, and then do a San Mai on it to strengthen it. An example:

You want a sword to be good against both fairies and werewolves, so you want both spectral iron and silver; but it should still be effective against people so we still need a hard steel edge. We start with three mostly-flat billets (one of each). Stack them Spectral - Steel - Spectral; and forge weld that. So now we have a single sandwich billet where the core is hard steel and each side is the less-strong spectral metals. Flatten that enough to be the blade. Then we make our taco out of softer steel (to better absorb the blows), and inlay the other billet such that it still sticks out a fair amount. Forge weld that together, and grind down the blade and harden/temper the construction.

You now have a blade where the cutting edge is hard steel, and flanking that are your two varieties of spectral metals. On a solid chop or a stab, you'd get contact on all 3 metals; but most of initial impact shock should be absorbed by the hard steel and the soft shell. Super effective if you don't need the spectral metal to be the one material penetrating.

If you do need that, you could still use the san mai technique, but rather than forging one sandwich, you could instead use different metals at different chunks of the edge. I.e. forge weld a few billets end-to-end so you have a very long billet that's e.g. Spectral silver for the first 6 inches followed by 2 feet of steel. Put that right in the soft taco. Forge as needed, and you end up with a blade that has 6 inches (either at the tip or the base) that's less durable, but super-effective. May work especially well for forging axes - a few inches in the beard inset for werewolf-killing, but the main belly of the chopping edge in hardened steel.

Don't make a sword.

Make a bludgeoning weapon instead, like a mace or a banded wooden club. Brittleness will be less of an issue.

If you really want a sword, there are those whose blade are heavy and not really sharp at all. They're designed to break armor; I refer to them as "aerodynamic clubs." Again, brittleness isn't an issue.

If you want to get really fancy, make one edge sharp (for nonmagical opponents) and the other non-sharp (for fairy werewolves). You need special fighting techniques for this, or at least be able to flip your grip in a hurry.