normal doubts
i think it is normal for professionals to express the hesitations they have and describe problems right at the beginning. During the first phone call it is up to the buyer to open the next subject. My saying this is not a recommendation for you to do this.
In your case it's how to anchor the foam e.g. by putting in screws here and there in all the right places.
Then, the discussion can focus on the following subject, which is who expressly warrantees that this portion of the preparatory work is sufficient and appropriate... and once again it is the buyer who has to open this subject so that the professional is not seen as shirking his responsibility. My saying this is not a recommendation for you to do this.
Retrofits and remodels are often a huge hassle to professionals; the jobs are smaller, more complex, with more unexpressed hidden expectations, and even the buyer may be unable to understand what the issues and concerns are. It can be a hornet's nest so a screening process goes on where they practically force you to insist you want them, you need them, you will pay them and whatever parts of the work they don't feel comfortable with you will do once they guide you a little bit. I have been in this situation myself more than once, and this is how I did deal with pro's who acted the reluctant bride part. My saying this is not a recommendation for you to do this.
In some big hardware stores you can buy the professional sized spray foam gun kit with nozzle cleaner. My saying this is not a recommendation for you to do this.
david