Right now we've had a non-binding referendum.
This has triggered a clause in the renegotiation that we did earlier in the year for all changes to be scrapped.
Other than that we're as much in the EU as we were before the referendum.
Once article 50 is triggered we then stop taking part in any decision making.
In terms of changes to the leaving process whilst the leaving process is in place they would have rather a hard time making that stick. Even if they did change it I'm 99% sure that those changes would not be applicable to a nation already in that process. Those nations would use the text of article 50 from the date that they triggered article 50.
Not vote.
Veto. A veto means that you can reject something. If a bill is vetoed by any nation with that power then it cannot pass.