Sounds like the issue is not whether to seal it before or after, but whether to seal 1x or 2x. Or use grout release before and sealer after. Usually the use of "sealer" before grout is to ensure that the grout doesn't stain or settle in the cracks of the tile/stone/pebbles etc.

You could do your own grout test and mix a little grout and see if it stains or doesn't release from a pebble sample -- that'd tell you whether pregrout sealer is necessary.

But I'd defer to Bill V.

Alot of times people will use the same sealer they finish with as a grout release, as well, and there would be no problem in doing that with pebble tile/ river rock, or what ever you want to call it.

Also, I just saw the second part of your question re: grouting. Again, I'd defer to Bill V., but I know that there's sometimes problems with grouting the pebbles because a grout float won't really work, bc the pebbles aren't flat. If that's the case, then you can:

1)spread the grout in the gaps by hand (fun!)and remove the excess with your fingers; or

2) put the grout in a gallon-size ziploc bag, seal the bag and snip off a tiny corner, and use the grout like icing and the bag like a cake decorating bag, and squeeze the grout through the bag corner into the deep gaps, then wipe off the excess, again with your hands, or whatever implement works. You just want to get the grout deep into the cracks, because you won't have the pressure of the grout float to drive the grout in. And it might help to mix the grout a little looser (not runny, but not stiff either). Good luck!