It seems that all marriable characters friendship levels will not go up past six smiley faces during the main gameplay. You can max the bar out at six ((meaning its almost at seven))but it wont go any farther so I suggest not wasting your items on them in hopes of going any farther as it wont work. I assume you'll have to beat the main storyline first.
As far as unmarriable tonwsfolk go i have a few almost at the seven mark so we'll see if we can proceed any further with them once i can find out. Hope this helps!.
You can duplicate delivery items by accepting and cancelling certian Bulletin Board requests. The requests which will work with this trick require that you receive an item during the job itself, typically for delivery. Simply accept the request, pick up the item from the one who posted it, then cancel the request without completing it; the item will still remain in your inventory! Repeat as many times as you wish. You can then sell these duplicated items for profit.