Hey ace - I see you've gave us a nice detailed review, so let me try to give you a helpful answer.
In general, selling sex is not the most well ordered business, so there are no hard & fast answers to your questions.
The answer will vary depending on the girl or agency in question. And even if we're talking about the same girl, it's going to vary depending on how she's feeling, whether she clicks with you, and whether she has a consecutive booking lined up.
Going by reviews, quite a few punters get lucky. Independent AW girls will let them over stay & have free time for a shower, sometimes even 5-20 or so extra minutes for the sex itself. But that does seem the exception, not the rule.
My own recent experience in London has been only with the House of Divine agency. (HoD)
With that agency, the clock starts ticking at the time you've booked. (regardless of when you walk through the door.) Exception - if the girl is late coming through the door, then the clock might not start until she's actually with you. But only if she doesn't have a consecutive booking (or they need the room). If that happens then you lose out.
Regarding showers, at least in the case of HoD, it should be no problem if you arrive a few minutes before the booking. Then you can have a shower without it coming out of your booking time. (Unless you are unlucky & other punters (or a girl) are hogging the shower, though some of their locations have two. ) You can probably have a free shower afterwards too if you want.
If you want to be safe though -ask the maid about showers when you make the booking. This would apply to all agencies & independent girls.
PS - just so you are aware, HoD in general does cut short your time if you book a busy girl, and lots of punters here don't recommend it. Just mentioning as its all I have recent experience of in London.