This is generic information, not legal advice to you or anyone else.
Only with probable cause.
Relax and wait. STFU while you're at it. If they ask for consent, decide whether to give it. If they don't ask for consent - because they believe they have PC - then there's not much you can do at the scene. Actually, I can't think of anything constructive you can do at the scene if the officer believes they have PC.
Decide whether I wanted to consent or not, or give limited consent.
"Dude, I don't have much time, so you can look under the seats and stuff if you want." You do have the power to limit the scope of the search.
Mostly, I'd politely decline.
Then, if you are unconstitutionally searched, you STFU and call a lawyer when it is over.
I think this is very bad advice. I've never heard of a limited search. Once you give them access to the vehicle then the whole thing is their baby to search.
im sure you'll show me some legal Mumbo jumbo proving me wrong but if they're in they're in.