Greetings,
1) in terms of legal equipment, MWM and Turret Mounts are treated in identical fashion
2) all allowed. Please refer to the clarification in the other thread
3) again per clarification - the prohibition on mount-inside-mount includes all modular weapon mounts, AP weapon mounts, squad support weapon mounts and all turret mounts
4) you are remembering an ancient BattleTroops-vintage rule, or a thread referring to that. This no longer applies to the current rule set so the answer is: no, not legal
Best,
Joel BC