Many of them are (reported here, use the search function), but not all.
Except when it doesn’t work: deathrattles, bench ‘reinforcements’, immunity cases…
Actually, the trio of Xuen, Chi-Ji and Niuzao are the most useful — and arguably used — of them.
Yeah, except that they don’t need to be upgraded, apart from Genn, which has other uses: working with Geddon (a very important merc for a number of approaches), summoning occasional singing girls and also healing.
Not quite right. Elise benefits from upgrades, what’s more, she’s been playable in an Explorer party, which is similar to Celestials due to their treasure (‘Teamwork…’). Drek doesn’t do much without upgrades, as far as I know. As for Malchezaar — it’s currently unclear whether he’s better off upgraded or not, considering the survivability factor.
Really? That was an annoying first boss, if memory serves — most of them are at such levels, requiring a lot of rerolling and so on, which hasn’t seemed worth the time for me, so I switched to skipping these, instead of completing them all, and doing easily farmable ones occasionally.
PS You forgot Medivh… There’s also that annoying Hooktusk for several tricks, but it’s currently unclear to me whether this one is worth upgrading.
I’m not mentioning some options that were quite playable at lower levels, but seem to be falling behind in terms of scaling.
Oh, there’s also Nemsy (mostly the ‘peaches’ path and ‘Can’t touch this’), possibly Aranna (she needs those HP at least) or other situational mercs.
Ironically, I don’t think lots of upgrades help against annoying first bosses, unless you ‘invest’ — or rather waste — millions of dollars into this bad design case, since those ‘+9000 shadow damage’ anomalies are gonna just delete your party anyway.