Since this is a well thought out honest question I'll go through it in some detail.
I'm not going to check your equation, because it seems a bit too complicated
How I approach these side pot calcs is to break it into two different calls, which it appears you sort have did.
Here you do not have any cost associated with calling for the main pot so:
EV_main = Eq_main * size_of_main
EV_side = EQ_side * size_of_side (after you call) - cost of call
That's it. Then add those up.
So, you give the first all in villain a range - you said it's wide so let's say it's top 25% of hands. Now, you give UTG a range - eliminate the top 10% of hands as he was a limper, and say his range is 10%-30%
AT, 78, (QJ, Q8, KQ):hh, TT).
EQ_side = 0.3745
EQ_main = 0.31288
EV_main = 18.97
EV_side = 0.26
EV_total = 19.22
So this is a hugely profitable call based on those ranges.