I did a search but didn't find a clear answer for these questions.

I think I'll need to get a standard Y award to either EZE or GIG around 8/1. Availability is scarce or gone (like Y1 all the rest zeroed out). Is it possible to waitlist the seat, even for 24 hs? (may be using the wrong credit card # trick?). I think the usual 3 day reservation is just for saver awards.

For these flights Y is almost sold out, but there's decent availability in C. Can I waitlist for a saver C award (which costs the same as standard Y) right up to the day of the trip? Of course, I would be willing to pay another $100 fee to move from Y to C. But 150K for standard C is just too much.

Beyond the lack of capacity controls and E+ for non-elites, is there any other difference between saver and standard? (for example in the priority for standby?).

I now have a saver award that I'll need to cancel in order to book this new trip, so this is sort of a change. Do you think if I call to have my miles redeposited and book the new trip at the same time I can have the $15 fee waived? (my new itinerary will likely be an open jaw so I can't book it online ).

It's been discussed here that if I book, say, EZE-IAD-SEA with no stopover, then if I want to make a stopover at IAD I have to pay the $100 fee (even though the routing remains the same). How does it work the other way? Say I book EZE-IAD-stopover-SEA. Can I later change flights so that in fact the stopover is eliminated, or will united, if the stay at IAD becomes less than 24hs, assume that the routing has somehow changed and charge me the $100?

Last question. Right now United only flies to IAD from EZE. Does that mean IAD is my only possible stopover? Or can I make a stop at any point on an allowed routing (EZE-IAD-ORD-SEA is allowed, so would I be able to make a stop at ORD?).

TIA