Need to schedule an onsite, the offer deadline is this Friday. It doesn’t look like I can manage to flyout before Friday for the interview, but trying. Asking for extending the deadline is difficult as it has been pushed pretty far during negotiation.
Since there is a visa transfer, how do i go about doing this safely?
No safety or guarantee with visa transfers. It's unethical to pursue two visa transfers but in the current climate it's probably wise to have multiple options. Don't worry about potentially burning bridges right now.
If both visa transfers go through then you can offer to pay up the legal costs for the company you ditched. Companies usually don't accept it anyway but you will come out feeling less guilty.