I'm an international owner and have bought and sold
DVC contracts before and its not really a problem with witness' I got my two daughters to sign it
before going to the local notary.
You might need to get a real witness if you take the cumbersome road and use the notary at a U.S embassy. But I've never used that. For my first contract sold I was on vaca in the states and I used a random bank as notary and they provided everything free of charge - great service.
For the rest I used a local notary in my own country and then afterwards went to get an apostille and sent everything to the title company. The local notary dont care if anyone else signed the document before coming as long as I didn't and they also dont care about the content that i'm signing, they only certify my signature.
I will be so bold to say as an international I can get my documents ready just as fast as any U.S seller, at least if we are talking days - hours not so much. When I've receive everything from the title company I can have everything ready in 2-3 week days (not counting weekends)
I assume notary rules differs from country to country but since your sellers are international they might be able to do the same as I did.