I tried the left foot left corner visa versa thing. I noticed no difference at all. In fact, I put a left footer to take right side free kick and it said he was uncomfortable taking it on his weaker foot so I still find it confusing.

I do know this. Some players are better than others and differences or similarity in skill points don't make that much difference. I do believe some players in certain positions are better than others.

I have 2 right footed centre mid free kick takers who are lethal, both have double figures and I only play one at a time. I only have 1 corner taker who is my AMR/ST right footed striker who takes from both sides and he has over 25 assists.

I wouldn't find the set piece takers from defensive player, I would keep trying attackers. You will have to try a lot of friendlies against weaker sides and keep an eye out on how many set pieces you have and how many he converts. If he has an average performances and misses 5 free kicks, he's not the one.