The 3rd tiler said that self leveling was not good on radiant floor and that there could be issues...
The first thing you do is stay away from that guy - he has no idea what he is talking about. SLC is one of the primary state of the art recommendations for this type of work.
Being on a third floor the cracks mean little. They are probably very stable and are a result of structural shifting over the years. They probably radiant from something having to do with the hardware and piping used to install the heat system. They are not subject to subsurface fluctuations in soil moisture-content and other stuff like that. If SLC won't work then nothing else will either, especially tile.
I can tell you that Schluter does not "cost"...it "pays". But I won't try to sell you Schluter even tho it could be the right thing to do here.
NO WAY !!!
He then recommended lathe and plaster mesh laid on the floor and then a coat of quickset put over top.
Not if he is suggesting a gypsum product. Gypsum and tile DO NOT go together under any circumstances.
This brings us back full-circle to what was used originally. Typically in a wood structure and on subsequent floors above grade a lightweight product like Gypcrete is used to keep the weight down.
Also typically when that type of heat is used above grade in a wood structure a product like Gypcrete is used to embed the heat system.
There's two votes for Gypcrete (or a like product) existing there
NOW, the problem if it is Gypcrete is that the floor requires special attention if it is to be compatible with a tile installation. The tile MUST BE isolated from the lightweight product.
Now we are back to Schluter DITRA again.
To repeat...it doesn't cost, it pays.
Besides, How big can this room be anyway? 150 square feet maybe?