@Reacon Well basically the Opressor MKII is a Delorean (what's the GTA V name again?) in fly mode. I'd look into the flags section of the vehicle item in the vehicles.meta and the strModelFlags and strHandlingFlags of the handling.meta. I've never bothered to dive into it, though.
@warfairhawk Usually every mod comes with a README which contains installation instructions. How about you read the README? Moreover this very forum section has several pinned threads which deal with mod installations. Open your f***ing eyes, willya! smh
@GambinoYT Until then you can use Simple Trainer to access the interiors.
@NicoXoN On a more serious note, look if any of your script mods (that's also trainers) makes you invincible and turn it off.
@NicoXoN Try this: Place a sticky bomb right at your feet and trigger it. 
@calzeus Hi there, from your words I guess ImNotMental has made some breaking change in his INMNativeUI.dll (they are from different dates, are they?) and so it doesn't work with both mods, which are of a different release date. Sigh
The only thing you can do is to forget about IMN's dealership. There are others from other developers. Try one of those.
@Fonespiders That's exactly how I do it. It takes up pretty much time here because mainly I have modified vehicle.metas, handling.metas and my own patches and I (we) have to be very careful to not mess up something. Yeah it's a pita but that's how it is.