1.  yes, not necessarily, and the threat is always there.
2. it's always a possibility, if it happens, you are busted
3. yes, they can.  will they, not likely, but always possible
4.  of course it's possible
5.  well-reviewed providers are less likely to be in cahoots with LE, as it may well be their only source of income.  however, as with all your questions, the possibilities always exist.

You just have to stick with well-reviewed providers who are reviewed by experienced hobbyists.  If you do that, the chances of you having problems with Uncle LEO are greatly reduced.

Good luck,
Swim

Try any scenario you can think of. . .

The answer will be the same. If you follow the advice here, you lower your risks. But your risk never drops to zero. Any scenario you can think up has at at least a tiny chance of failure. If that level of risk is unacceptable to you, don't hobby.

There's always a slim chance of LE going after providers - and that goes for the famous well-known ones.

If the hobbyist and provider are inside the room, the police cannot go in without a warrant.

And if they do go in,  what proof do they have if they have not witness an exchange of money?  You could just be boyfriend and girlfriend.

More reviews means = less chances of LE.

You worry too much.  Just do it.