Would they be comfortable installing speakers if I source them myself?

They installed speakers I purchased from Bavaudio in the stock locations in our BMW. Ryan took a look at the parts before committing to the job.

Yes

Yes, but if they don't work properly or don't fit well or blow out in a few days you would be out your install money. If you buy from the installer you won't have those problems.

Yup. Ryan has installed my own equipment in 2 of my cars. Just gotta file your own warranty claims if there's an issue with the items.