What if you chose the place. --> You should be safe.  (E.g. The police could not pre-stake out the place do set up the equipment, or just the back up cops they need for a safe bust).  
What if you met at the bar.  --> Hat wastes a lot of police time which they don't like to do and it looks very bad to have a criminal dining and drinking with a law enforcement officer.
What if you took time to chat and get friendly.  --> same thing, cops don't have time to make friends with this type of bust.  Think of traffic cops.  It is a mill. One after another.  They want a room set up so Johns can come in every 30 minutes to get busted.
What if you just set up an incall location?  --> you control the environment, even if at a hotel, and cops don't like to be alone with criminals, even naked female perps!!!

I agree with your first point. LE is probably not going to take the time to wine you and dine you just for a bust.
As for the second point, it wouldn't matter if you had dinner, went dancing, saw a movie and then spent half the night looking at old vacation photos before you did the deed. Once you accept the money you are subject to arrest.

but I suspect a large number of hobbyists would prefer not to for reasons ranging from time (and whether or not it's on the clock or not) as well as just prefering the more typical wham, bam, thank you maam type of encounters.

Try it out on new clients and let us know how it goes.

Lisa,
Would this drink time be on the clock or off?  Many hobbyists cannot afford the extra $$$ you might ask for to have a pre-date meeting.  If it was off the clock, I would have no problem with meeting you like that.

Like was said above, I think it unlikely LE would go to this trouble to set you up, just to bust one provider.

Swim

A mature provider advertises a $300 session for three hours, the first two of which are at a restaurant or similar location.  The reviews confirm that is her procedure.  However, she has a full-time job and is not relying only on her providing income.  

It is reasonable to assume that LE may be unlikely to invest two hours in a pre-session during which the lady might ask enough pointed questions to become uncomfortable and cancel the "date".

Re: What if

I have found this to be a very good way to meet someone new. I feel much safer after a meet and greet. I like to meet with a more mature gentleman and I think that they like the time to get to know each other too. Time is usually shared.