Re: understanding digisenders
Yes you could use it in another room with the TV turned off, it is taking the output from your freesat box so the TV downstairs doesn't come into the equation.
For changing channels, this depends on the digisender you buy, you need to make sure you get one that has a remote sender. This is normally a little cable/eye that you sit in front of you freesat remote sensor so when you use the remote upstairs it will send the remote signal via the digisender.
Yes you can use two remotes.