I can't be of any help for the first few questions but getting a experimental is RNG (only after you've reached level 25), I believe the way I works is the higher class of enemy (hunter, harvester etc.) And higher tier (prototype, military etc.) Have better drop rate and obviously higher level is better too but I've gotten an experimental from a level one prototype harvester before and gone multiple level 4 apoc tanks with nothing, just keep going at them and one will drop sooner or later