Different world, different rules. Polygamous relationships are a thing there, and Erza doesn't really have a good concept of relationships. That said, Mira technically isn't part of their little group yet. I honestly, don't even remember what happened at the chapter you stopped, but even in the latest ones, she is mostly testing the waters.

lent for a time and not given away. you could even think of it as him storing it in a safe place

it might also lead their enemies to him

any*

if he can produce that much a day

such a reward would remove and tension. I'd much rather the mc earn connection with powerful figures or become strong enough others wouldn't want to mess with

I can't say that it's a good business fanfiction, but I will try my best. The main character will also learn magic as well and possibly go on an adventure or two in the future when his shop stabilizes and gets some employees. What I'm saying is, that it won't focus entirely on the store.

might be a little bit cheaper, but it's pretty much the same

a futon is pretty much the same price as a mattress

Yes, probably more than one.