I recommended 3.5 as for some reason I recall to go a bit heavy on aged beers. I think I read somewhere that over time more co2 comes out of solution in the beer, so you overcompensate a bit. So I think if you do 3.5 you might end up near 3? I could be wrong on this and remembering it incorrectly (hopefully not opposite!), but either way 3.5 is nice if there is no drop.
The previous brett may be enough, but I wouldn't take the chance, hate for the beer to end up flat. You could pitch more brett if you wanted.
If you use the dry champagne yeast, a half packet should be fine.