[Yamamaya42]

First and foremost, the loss of L42 is not going to mean a catastrophic end of this set, YES, there is no replacements to be found on the market, but this is nothing new to people who restore vintage electronics, and as I recall there is at least one person (if not more) out there who has the equipment to wind their own replacement coils, who I no doubt would be more than happy to help you out in replacing this coil.

Also, as been stated several times, you have yourself severely hamstrung, there is only so much you can do with the limited tools you have on hand, digital multimeters, soldering irons, a schematic, a handful of parts can get you only so far, complex issues like you are having need an oscilloscope and the general know how to use it, otherwise you will keep hitting walls, over and over and over.

You have been given tips by Squirrel boy and myself to try and protect L42, it's your choice to use them or not, it would be in your best interest to do so and turn your attention to the tuning / IF problem first.
Even if putting in this protection degrades the ability to focus a bit.

Things I have been wondering.
What prompted the replacement of the FBT in the first place when the original seemed OK?
This is an extreme measure, and only done when you are VERY sure it's bad.

What caps were replaced in the set?
In this era there are only 2 known that will be problematic.
Electrolytic capacitors and any obvious paper capacitors.
ALL ELSE should be left alone unless you can show that they have failed.
These include all ceramic disc capacitors and sealed film types (drop caps).
To arbitrarily replace the above w/o reason (proof of fault or fail) is asking for a major headache.

The way choose to go on from here is yours, but your best choice is to protect l42 as mentioned.