Hi there, I'll address your points directly so it's easy to read. It should be stated that any vehicles, weapons, visual mods and other GTA V mods should always go into your mods folder.
1) Replacement vehicles, such as police, police2, police3, fbi etc. should go into mods\update\x64\dlcpacks\patchday22\dlc.rpf\x64\levels\gta5\vehicles.rpf\
2) The lights likely do not work because you need ELS for some cars, which adds more lighting functionality such as additional light stages etc. You can get it here, please read the installation instructions. Don't forget to download ScriptHookV too (not ScriptHookVDotNet). Links:
ScriptHookV: http://www.dev-c.com/gtav/scripthookv/
3) Yes, you can either replace a different vehicle slot such as fbi or fbi2. Alternatively you can install an add-on version of a car, and configure that into your backup system, either by editing the default LSPDFR backup.xml, or DefaultRegions/CustomRegions.xml if you use the UltimateBackup plugin.
4) Is that happening with all callouts? Or specific ones? It could be an issue with a specific callout pack. I'd suggest testing your plugins one by one to see if it happens when you add a specific plugin.
5) Tow trucks can be called in using external plugins. I'd strongly recommend BejoIjo's StopThePed, which adds a lot more functionality other than tow trucks. I'd also suggest looking at his other plugins too. You can find his website here, he hosts his files on there: https://bejoijo256.wixsite.com/bejoijo
6) OpenInteriors usually works by standing in the coloured circle and press a key to enter. If you can't see those, I'd suggest re-checking the download page for installation instructions and requirements - make sure you've installed it correctly (including any dependencies).
If you need anything else, please let me know.
Thanks,
Zen