All of the SMs are beatable under the right circumstances. Borys and Dregoth are the real challenge.
They were 40+ level in 2E. Hamanu 43, Nibenay 44, Dregoth 49, Borys 50. 2E DS used a different way on counting and counted levels of psion and wizard once, so Borys was “level 30”. But his real levels was wizard 20, psionicist 20, dragon 10.
I don’t think you can add the CR if SM’s templars unless they are part of the fight. With my versions, a good rule of thumb is that the CR is their character level + number of salient feats + number of ranks of metamorphosis. At high levels CR tends to break so guessing is all you can do.
They gain XP slowly, but generally not through combat. Lucky for the PCs because of the SMs could XP farm they’d be dead meat.
They were 40+ as I explain above. The templarate had the SMs at 50 - 80 levels for 3.5. I’ve got them at the same level they were in 2E.
The archfiends/demonlords that were published are not necessarily the true forms of those creatures, but lower powered aspects. Check out The Gates of Hell , by Dicefreaks Publishing for what a real archfiend looks like.