1) The main reason you can't get the Caliban at chargen is due to its DR of 7. Also, someone somewhere with some amount of authority ruled that the mount benefits from the Used price, but the commlink/deck does not. So, you would get the price discount on the 5K mount but not on the commlink.
2) Yup
3) Not sure on that one. I would think it would be all wireless. With that said, I think a lot of us have the understanding that there is some kind of access hatch to get at it simply to eliminate the pain in the ass of significant surgery to have an installed cyberdeck repaired after suffering matrix damage.
4) You already get a DNI from the installed commlink, but the noise reduction is def handy.