is hired merchant perm?

if I use a hired merchant will it still be in my inv?

 

Yes. It doesn't expire. You just need to reset your store every 24h.

 

if 24h pass after I open my store will it disappear?

 

It will. Your items will be sent to fredrick and after you retrieve them you can reopen your store. Alternatively you can just reopen the store before the 24h mark to not lose your spot.