they should be smarter and not collide with creeps or demigods for starters. They just get stuck everywhere and all of them can't even attack at same time.

Make creep enhancing items % based.  Right now Erebus and Oak gets +4 damage (for example) for all 1,000,000,000 of their minions, whereas Sedna and QoT get teh same bonus for only 4.  The minions are the same strength roughly without upgrades.  I could see stock Yeti's taking on stock Nightcrawlers.  But after Erebus gets +72 damage for his 18 minions combined and Sedna is only gettting +12, it really starts making a difference.  Making all creep upgrades % based is the only fair way to do it.  That way Erebus' minions would be getting +4 (4 damage * 18 minions = 72 damage boost) and Sedna would be pulling about +18 (18 damage * 4 minons = 72 damage) from each Yeti.  The math is assuming damage is atleast comparable at 18 nightcrawlers vs 4 yetis.

This is especially true for health.  Erebus getting a +400 health item roughly doubles his creeps healh.  While Sedna at final rank yetis only inches it along.

Erebus maxes at 10 Nightwalkers... just... FYI. Unless you're adding in the idol minions, then yes, 16, but then remember that Sedna jumps from 4 to 10.

Your argument still holds water, just saying your maths are a bit off.