Should I Do This?

I'd opt for Def Wave over Res, especially without DC.

Hell no, H!Myrrh wants to stack defense and couldn't care less about resistance. Also that's a perfectly serviceable IV in many base content maps to avoid natural doubles. Also he has bold fighter so I'd advise against foddering him for a sub-premium skill.

Probably not. H!Myrrh is focused more on Def rather than Res, although she has plenty of both. And since this build doesn't seem to have Distant Counter, it'd really only help Myrrh fight other dragons and potentially any Winter Tharja's you come across. I would recommend Def Wave from a recently demoted Silas to 4* instead of Res Wave. Since you say you have a Silas, I'd opt for Def Wave.