No, not without purchasing stems separately, and those are not official parts.
Not very, there are o rings you can buy cheaply. Alternatively, a Type S exists that does the same but from the factory. These keyboards tend to "whistle" and after many years of using a Type S, I have come to dislike it.
You can lube the board easily, but you do have to take it apart. It's a very easy procedure, although a bit time consuming. See a guide for that here:
https://www.mechblog.org/post/95009796509/hhkb-lubing-and-silencing-pictured-1-pro-jp-2Yes, you can lube the stabs.
The Hybrids ones can be remapped, the others can not (but they have DIP switches for some customization of the layout), not without an additional non-official purchase.
Depends on how much you value your money. For someone well off probably, but you can do it yourself easily in an hour or two.