Regarding friends of the opposite sex, I think that as long as their interactions are similar to interactions with friends of the same sex (where available), and quite different from interactions with actual romantic/sexual partners, the distinction should be fairly clear.
Regarding the age-gap friendship, I would suspect that as long as neither is performing any of the actions of grooming (e.g. one acting to isolate the other from people outside of the two of them), it should be fine.
And in fact, I feel like one thing that might help in all cases here is to either show or reference more and more-multifarious interactions. Show us one of the age-gap pair chatting comfortably with one member of the couple; show us one of the prickly members of the team tossing out a comment about a previous social interaction with a few of the others; and so on.
Further, have the group chatter amongst themselves when together, as appropriate: quite a bit of social characterisation can be had from the little things that members of a group say to each other when doing other things, I feel.
That said, in all of these things I daresay that there will likely be audience members who read into one interaction or another things that aren't there—but that's pretty much always the case, I suspect.
My Games & Writing
I'm working on a sci-fi action-adventure novel with nine leads, all of whom take turns as first-person narrator. They are (oversimplifying and giving them pseudonyms) leading man and Magnetic Hero Bob (age ~20), sweet White Magician Girl Alice (also ~20), who are together the work's Super Couple, The Big Guy and Boisterous Bruiser Charles (also ~20), The Lad-ette Action Girl Diana (24 or so), Deadpan Snarker Black Magician Girl Eliza (also ~20), Cold Sniper and former Hitman With A Heart Of Gold Frederick (about 28), (relatively) Old Soldier Drill Sargent Nasty turned Team Mom Gracie (early 50's), team spiritual leader/Badass Preacher Harry (late 40's), and Ridiculously Human Robot Kill Bot Isabel (technically about 2, but mentally 20's). I've done enough worldbuilding that I have a good idea of what each of these characters are, now I need to figure out how they all interact.
I'm admittedly not a huge fan of tropes like Love Dodecahedron and Dating Do-Si-Do (not to say those tropes can't be done well, I moreso don't have faith in myself to write them well), so I want to hold hard and fast to the idea that Alice and Bob are the only romantic couple between two group members. I also want to push the idea that the group of nine is a functional unit that's not falling apart, so I don't want there to be too many Sitcom Arch Nemeses and Vitriolic Best Buds. Then we get to The Friends Who Never Hang. Admittedly, with a group of nine it's inevitable that not everyone's going to get to share major screentime with everyone else. But at the same time I don't want to make it seem like the entire series revolves around Alice and Bob, who as the All Loving Heroes would presumably have a decent-to-good relationship with every other team member while the pricklier members, like Diana, Frederick, and Gracie, would possibly fall by the wayside as they don't have good relationships with the rest of the team. But on the other hand with Alice and Bob, I'm worried about having either of them form meaningful relationships with the opposite sex since I don't want people immediately jumping to the conclusion that the only reason they're hanging out with anyone of the opposite sex is to cheat on their partner with them. The problems with having an age-diverse team are also weighing on me - for instance, Token Relgious Teammates Alice and Harry would likely have a lot to talk about and would probably be good friends, but given their age difference I'm scared people will immediately jump to the idea that he's grooming her or something (which he wouldn't, and as someone who's had plenty of healthy, non-sexual relationships with people of vastly different ages from my own, it really annoys me that people immediately put the "#problematic" stamp on any Intergenerational Friendship).
I want my group to have some interesting Platonic Life-Partners and Heterosexual Life-Partners relationships, I just don't know how to make them work and avoid having anyone left too far out on the sidelines.
Koichi really steals? No dignity.