Let's take an example that I schedule an appt with a well-reviewed provider and she is working with LE (she got busted or something). But I only schedule a massage and I don't ask for anything sexual but if she were to start a hj/bj, I wouldn't resist. What would LE have on me in that case?

Second question, what can you do to make completely sure you won't get busted when seeing a provider for the first time? Schedule only a massage and just ask for a legit massage? I know if she's well reviewed, she's likely not, but I'm just wondering about a worst case scenario.

You may get off with a jury, but if things go that far, it's no victory.

The only good defense against meeting LE is to see well reviewed providers.

Meeting up with a legit provider who has flipped is rare, but not unknown.  Keep close tabs on your local board because often they get mentioned on there.

(still not a lawyer)

I'll answer your second question first.  You want the worst case scenario?  The worst case scenario is that you have wild and passionate sex, then later find out that she is really a man.  In my book, that is in fact the worst case scenario.  Worse than being robbed, and worse than being arrested.  

With that out of the way, I will answer your first question. As far as I know, to be arrested by LE you have to solicit sex, meaning you have to be the one to offer money for some kind of sexual service.  If you are offering money for a massage, that isn't illegal as far as I know.  

When there is a sting going on, LE busts people right after the solicitation for sex happened, not an hour later after all parties are done having sex.  

So if you are going there for a massage only and you get it, then you randomly get a hj/bj, I would say you still didn't solicit anything other than a massage.  If you are actually in the process of getting that hj/bj service, it would be safe to assume you are not dealing with LE, because LE doesn't take it that far.  That's just my opinion, others may disagree with me of course.

all you have to do is show up and you're busted.  It's considered an act in furtherance, because you responded to an escort ad.  Nobody is going to believe that you went to an escort to play poker even though I do (just kidding).  Most of the undercover ladies will try to get you to ask for a sex act for money.  If they can't get that, they will ask questions about what you want sexually.  If she's undercover, she CAN'T touch you illegally.  If she does and busts you anyways, get yourself a damn good lawyer and fight it.  Cops are crooked.  I won't lie to you about that.  

Second question is that nothing is fool proof unless you are able to obtain her personal information i.e. real name, etc.  Decoys will not give that up.  Anybody can be flipped if they have enough charges pending against them, and they are looking at serious jail time.  Unless you know that girl real well, you are taking a risk.

it's quite rare and would present some real issues for a prosecuting agency.  First, it's much easier, safer and cost effective for LE to use female cops as decoys than trust a working girl to do things in way that would be good evidence in court.  As to the idea that it started off as legitimate massage and then transmogrified into a blowjob, that too would be a tough sell for the prosecution, juries just aren't into law enforcement agents (sworn officers or not) actually having sex.  

As to your second paragraph, I repeat my TER advice mantra: Get a VIP membership and learn how to use it, ommm.  I've been doing so for over 10 years and haven't had a whiff of a problem.  Stay with well reviewed girls with recent reviews and you should be fine.  You will never be "completely sure" but you can reduce the risk dramatically.  This is also one of the reasons I gravitate to porn chix: if I set up an appointment with Trinity St. Clair and she answers or comes to the door, I'm pretty sure it's not LE.

where cops got massages & even had sex...  before busting the gal.  In most cases, it's not illegal for a cop to do so but made the agency look bad when it went poublic & a lot of cases were thrown out because the proscuters didn't dare.  
But countless others plead or were found guilty because no one would believe them.    

The whole point is to not end up in the same room with a LEO.  If you do proper research, it's as safe as we can make it.