My thoughts exactly!
I thought Maisyn was Raisin with an M. The first thing I thought of was Raisin. I would recommend against using it.
To be honest, I think phonetically Maisyn is pronounced my-sin. Not exactly appealing.
Me too. I wouldn’t necessarily name my kid Maisyn but whatever floats your boat. I also vastly prefer [name_f]Maisie[/name_f]/[name_f]Maisy[/name_f].
The Masons that I know are boys. It seems quite popular.
The first one I met was helping his dad (a stone mason) build a stone wall. I said calling him [name_u]Mason[/name_u] seemed like job coercion! The father said it was intended to be. 
So it’s not the spelling that bothers me, but the notion that you hope she’ll grow up to be a mason.
Why don’t you just name her Maise, another name for corn. Then ppl may call her [name_f]Maisy[/name_f].
Named [name_u]Mason[/name_u], she’ll probably be called [name_m]Mace[/name_m].