You can buy the pass at any point so there’s nothing wrong with waiting to see if there’s a sale.

Yes you’re pretty much always able to book seats elsewhere than on eurail.com. It’s even advised : less or no fee, sometimes seat selection, etc.

Have a look at https://www.seat61.com/interrail-and-eurail-reservations.htm and do not hesitate to ask further questions mentioning route and date. Some routes need to be booked well in advance (Eurostar for example), on some you can’t make reservations at all (trains in Belgium and the Netherlands) and everything in between...