[@Ceta de Cloyes] On first questions: yes, we are operating via more logical rules here and not simply the Game system logic here. Mediv was able to use both warlocky shenanigans and also pull typical magic, so yes in some ways a person do CAN retain some their skills if not all. For the examples you gave yes a warlock could summon water elementals, but depending on the situation some of their demonic summons are more useful for them. Or on the rogue demon hunter, yes sneaky demon hunter is quite possible. Demon hunters gain their demon hunters power on top of their previous occupation. Working off some of the tabletop rules here though, but it does make sense no? I mean if you've lived your life so far sneaking about and being sneaky you wont simply forget those skills. Deathknights are a different kind of breed in this sense cause htey were raised to be deathknights specifically so their skills will be replaced only. They are OP enough already.

 On the relationships question about summons: most lower level elementals from my knowledge do not possess high intelligence in the same manner most creatures do. In fact most elementals but hte few most powerful are mindless ones so it doesn't matter really what you summon. Any summoned elemental with the right spell would just comply with it's master. Demons are trickier, but for the purpose of the RP I will allow for them to be summoned and the ones contracted specifically. Let's just call it: temporal effect of convenience magic ^^ The only thing on warlock summoning demons is that it IS harder to summon their demons into the world.

 Think I answered for now ^^ feel free to ask more if you wish.