This cannot be mounted to a chair, but you could mount it to a floor or baseplate of some sort to anchor it. That would allow you to use it in a ground blind for your Crossbow. A better option might be their chair that already has it built in: https://www.opticsplanet.com/caldwell-deadshot-chairpod.html
Please also refer to the other answer under this question for a users experience with the product in this circumstance.
The height is adjustable from 31" to 47".
You can find the separate base section here: https://www.opticsplanet.com/caldwell-deadshot-treepod-additional-base.html
It comes with everything shown in the first image on the product page, it is both the mount and the rest.
You can disconnect the upper part from the mount to move it to the tripod.
Yes, additional bases are available, they are $48.79:
https://www.opticsplanet.com/caldwell-deadshot-treepod-additional-base.html
Mark H.
Yes, the height can be adjusted.