I think your understanding of the process is roughly correct.
Of course, you can't trust the ads here fully. Most are heavily PSed. There can be cases that the agent send you another girl. If that's the case, of course you can say sorry and walk off.
Assumed to be maximum 45min. Most girls would rush you out after you exploded before 45min. A few service-oriented girls might chat and/or massage you until time is up.
As to what you can do with her, I think the is that you can explode inside her vagina once within the 45min time frame. Beyond that, you can try anything as long as the girl doesn't refuse. Even if she refuse, paying her extra might change her mind. For example, I once paid $300 more to get the second explosion.
In general, HG is not a good thing for people who like the certainty of actually meeting the girl in your mind and meet expectation of a fixed level of service. If you prefer certainty, Macau sauna is a better choice.
Good luck!
