See your questions in the light of just one simple fact: you are not about buying a game, but a professional training tool designed by the needs and demands of the real military, and intended to compete against the hifi-simulator cabins the military has by super-aggressive pricing. This is what differs details and focus of SBP from that of normal game releases.
1.) Immersion is high, but it is however a diffrent kind of immersion than SH3 that was breathing the atmosphere of an ancient epoque.
2.) as you can read in my own review, only some tanks currently have detailed 3D interiors, namely the Leo2A5 which has the most detailed of these. However, at no cost I would like to need switching back from my HOTAS to mouseclicking in these 3D interiors, in handling it is too timeconsuming. You are not travelling in a sub with sometimes minutes of time passing by before youzr keyboard command is carried out, but in a tank - you may want to flip sights within splits of a second. DO NOT compare to SH3, tank warfare is totally different.
3.) I still wait for anyone complaining about the interface and keyboard commands being '"difficult". You can assign keyboards to your liking, btw. By far the best solution (and closest to the real thing, probably) is to have them on your HOTAS.
4.) Depends on the scripting. AI buddies will follow the lead tank, however at battle positions or depedning on conditonal route variables they may act indepednetly to counter hostile contacts, face incoming fire, find hull-down positions. AI is coinsidered to be very good in SBP, but the overall movement depends on the mission designer - and your own mission planning before playing. The player sues almost the same interafce for that like the mission designer. It is the same principles and mechanism at work. See my review were it is explained en detail.
5.) The sim has a mission editor, and a terrain editor. It already could make a diffrence to take a mission and change the vehicles for different ones. Currently there are three or four dozen additional missions uploaded at their download page. The functionality of the editor is extremely easy to handle, if the mission design will be good depends on the deoisgners tactical undersatanding and cleverness. It could be a stupüid gunnery range. It could be a highly flexible, mobile battle. It could be filled by surprises or not. Depends on the mission designer. I wrote the ditor is the best I have ever seen beside that of Flashpoint. I stick to that statement.
6.) Your hardware is fine.
Again, all your questions got dealt with in my review, and very detailed so.
If you want a simulation, this one is for you. If you want a game-sim with focus on the gamer's interests, be cautious. so far I haven't heared of a single guy who regret the buying. Only one I know who sold it - because he received a new assignement and had no more time left for SBP.
I also recommend the interview with Ssnake to be found in the ressources-sticky. He says some things baout the sim'S focus and future, and how it competes with the professional HiFi-Simulators.