i'd definitely want to tell my past self one thing, i even know exactly what i'd say. i have the benefit of hindsight and could really positively change the course of my life. with the future, well i don't know what i don't know. i wouldn't know what to ask that might help me.

for the 'who in skz?' part of your question:

chn - past.
i think he's want to tell him self to be kinder/gentler to others in his younger days. not that he was horrible, but he clearly regrets being more hot header when he was younger.

mnh - my gut instinct was 'neither'.
i just think he wholly accepts himself and therefore his past. so, i think, he doesn't necessarily feel the need to guide or support his younger self - he knows he will be fine. and i don't think he cares about the future besides what he is shaping for himself.
if you twisted my arm, i'd say future. but he'd ask his future self something very random/out of pocket. probably something like asking his future shelf to show him a photo of chngbin with all his hair cut off for the military.

chngbn - future.
another one i see has having accepted himself and his past, and not feeling the need to guide baby chngbn because he knows if he sticks to his principles he'll be fine. but i do think he'd be tempted to ask about his future. where his career is, who he is with. something like that. i think he's a romantic at heart, so probably something about that. but maybe just something sweet about if his family is happy/healthy, i think he'd be comforted and happy to know they were.

hynjn - past.
this one was harder for me, and i made it a bit sad in the end :/ but i think hynjn has suffered from being under public scrutiny, thinking about his suspension... i think he would be tempted to warn his younger self, or at least comfort them and let himself know it would be ok. he get to be with his found family again, that they held that space for him while he was away.

jsvng - past.
i think the teaser we got for the hold my hand mv answers this enough. nodding at a younger version of himself, as if to tell him it's all be ok? yeah... i think he would want to tell his younger self it would be ok too, that he has good people that will look for him when he is lost. or, maybe, he'd tell his younger self to be kinder to himself.that he wasn't a volcano hurting everyone around him. or that if he's an alien its ok, he's found his planet and his fellow aliens. <3
honestly if i think about this too long i'll write pages about this....

flx - past.
i think he'd just want to comfort his younger self. tell him it would be ok, he'd settle in and learn korean and get through his adjustment period. i think, maybe, that he'd tell himself to trust chan. tell him to look for him, that he will always be there for him...

svngmn - future.
another tricky one for me. but while i see him as introspective, i think he'd be quite pragmatic and ask how best to impove his singing. or something else like that.

imie - future.
again, i see him as kinf of pragmatic. so i think he'd be focused more on guiding the course of his current life, than editing what has already happened to him. i think perhaps he'd ask if skz was still going strong, his found family and support system <3 (interestingly, i wondered briefly if chn would be tempted by this option, but i think he is incapable of envisioning a world where that isn't the case)