Do Iv's affect stats gained in merging?

No they do not, it's always +1 to two stats per merge, no more, no less. At max merges of +10 it's +4 to all stats.

It might affect the order in which they grow, but not the amount by which each one grows.
The growth order is(based on their stats at level 1) which ever stat is the highest, to whichever stat is the lowest. So if the characters stat line to start is 20/10/3/5/4 the first merge gives HP and atk, the second gives Def and Res, the third, HP and speed, the fourth, atk and Def, the 5th, spd and res. Then the pattern repeats.

If there is a tie, something like 20/5/10/5/7, it goes in the order It goes from left to right.

IDK for sure, but from what I've seen, most merges go like:

+1 - +HP and Spd/Atk
+2 - Spd/Atk and Def/Res
+3 - HP and Def/Res
+4 Atk and Spd
+5 Def and Res

But that applies to power/speed units, which are more my style.

It does affect it just a little, whatever is the highest stat it will go with, but when +10 it will always just end up being +4 in every stat.