Paladin resurrect ability and no-loss
 

Does it count as unit loss in the end game score if you lose a stack of units but are resurrected 100% afterwards by the paladin's ability?

Yes they count for losses, because those units `died` in battle and got resurrected by divine intervention after battle :-)
Paladin is really hardest fo no loss, because he lackes any clear tactic to overpower enemy army (warrior huge rage bonus, mage double cast). Only thing paladin gets is bonus % for resurrection slepp/paladin prayer so you`r kind of locked with army up to 4th lvl, and mass ilussion on paladins :-(

And I've already spent runes on lvl2 resurrection :(

It`s not totally wasted runes, with every lvl of resurrection skill, you get more % for resurrection spell, and paladin prayer. Not much but it`s something, but it`s always easyer to cast 1 more illusion, then spend bunch runes on skill just for that :-(
Have fun playing.

But just to be sure, summonlings (royal thorn spawns, phantom image, demonologists summoned demons, etc) do not count as loss?

No ofc not :-)
Only units you start battle with shouldnt losse any numbers. Anything you cast use during battle wouldnt count toward no loss (summons, illusion spell, hypnotise, sacrifice+resurection...) So ifd you want no loss, you must end all battles with same number of units as before battle (or more, in case off sacrifice+resurrection).

What... You are horribly wrong, if some of your units die in a battle, but are resurrected back to the full initial stack by the end of the battle, they do not count as a loss, not for the final score screen...