My partner and I (MF 22/23) go a lot to both beaches. We were actually at sunnyside last sunday! Here's some responses to all your questions hahaha!
1. pros and cons with both. Heaps more space at pt impossible but it's also a massive drive since we're SE melboune. Parking at sunnyside is super limited meaning a walk up/down the hill on a nice day is likely. Parking at pt impossible is a lot better but it's down a bumpy road so my toyota 86 on lowered springs absolutely hated it. Our preference is pt impossible for a day trip, sunnyside if it's an afternoon only!
2. Sections I haven't noticed but ... possibly? Usually it's a mixture of exhibitionists/naturalists, regardless of sexuality everyone appears to get on. I wouldn't worry about sections just set up where there's space and you're an even distance either side from where others have set up.
3. I'd say it's very much personal preference what level of social interaction people want. In the water I'd maybe? say hello if I was nearby or and wanted to complement a tattoo but I wouldn't be walking up to people trying to relax next to their gear. Also depends on who you're talking to - if you're going to approach another couple do it together but i'd be sensitive to the fact that people find nude beaches to be a very vulnerable space. Power dynamics play into it a lot and could potentially get you in a lot of trouble if you're overly sleezy.
Making friends at the beach is likely going to be difficult for those reasons... but as you've put at the end of your post maybe arrange to meet people prior! We had a first date/meet with our best friends who we met off Feeld at sunnyside & it was a fabulous time!!
4. Probably best to avoid arousal. It goes back to point #2 and #3 - many people are naturalists and don't really find it to be a sexual thing. Some people get involuntary erections which is fine but it's not a good look to be walking around showing off a stiffy. Lay on your front or go in the water - they're good ideas.
5. Sexual encounters do happen - more so at pt impossible from what i've observed. It's one of those things that everyone knows whats going on but probably best not to stare & turn a blind eye. Dunes at Pt impossible are common areas for people to go. To put my lawyer hat on: having intercourse on the beach or in the surrounding areas could get you an indecent behaviour charge. If you absolutely cannot wait until you get home stick to the car or find a very secluded place.
I think that covers everything? Feel free to reply/dm and ask questions you have any or possibly if would like more friends to go with! :))