They can't prevent the establishment of a new league - however if they don't sanction it the players will not be eligible to play in any WR (World Rugby) sanctioned event (international rugby) WR (World Rugby) also would not supply referees therefore they would also need to become rebels and also give up on any international representation. WR (World Rugby) may also ban players, referees, coaches, administrators for a period of time maybe permanently like they did with league players in the amateur days.
Pretty much this.
In terms of setting up a "rebel league", I think questions like insurance and the rules could be overcome.
The issue really comes down to the willingness of the participants to cut ties with rugby union. Andrew Forrest himself doesn't have too many ties to worry about, but the others in a prospective WSR do.
As you say, players, officials, administrators, etc, spending some time in the cold.
And moreover the teams themselves as currently proposed: from Japan, Fiji, HK, Singapore, et al., (either affiliated to their national unions or owned/controlled by said unions) are not going to quit World Rugby.
They don't need to, anyway, if WSR kicks off without Aus/NZ. Simple enough, I reckon.