You're on the right track, did you fully undress her first?
To grope ass you simply put her in doggy style position and choose open hand to grope ass
Yeah, it's still early access and unfinished game, dev focuses on adding new stuff rather than to adjust and fix the existing ones
Same thing with suspicion gain despite having max relationship or wake up Yuna and bang kaede standing from behind... The game needs some polish which will come later
Cheers