I'd second most of that, Kerzerho is a must and, if you have the time, doing the big circular walk to pick up some of the lost sections behind the surviving alignments.
In terms of sneaking in, if you go in the evening you probably can at worst you'd probaby get shouted at. Kerlescan is easier as a bit less exposed. The alternative is to save your trip till winter when quite a few sections are open or go on one of the guided walks that run a few times a day.
To be truthful, if you go to Kerzerho and St Pierre Quiberon, it's not tooooo dissapointing to not get into the alignements.
You also HAVE to go to Kercado, one time holder of the oldest megalithic structure in Europe title, as it's great and if you get there before about 11:00 you'll have the place pretty much to yourself.
Jane's excellent blogs cover the trip I took them on and would be relatively easy to replicate with the blue 1:25000 IGN maps and a good guide book.
BTW Gavrinis can sometimes be a bit off about photography inside. They said I couldn;t use a flash and I've heard stories of people not being allowed to photograph at all.
If you're after a cheap clean basic place to stay you can to a lot worse than the Formula 1 hotel in Vannes. They're a real bed in a room affair full of an odd mix of families, businessmen and builders, but the Vannes one is within walking distance of the town centre. It's where Julian and I stayed on the TME trip.
Any more advice required, just ask!