You would use the existing power adapter i.e. that came with the Librem 5. Unless you already have a bigger / better one.
In theory yes but in practice that port may not provide enough power for the dock plus the phone (if you are actually plugging extra devices into the dock).
A USB-C port of a laptop with power delivery would be much better.
I understand your concern.
My recommended approach is to plug e.g. a keyboard into the dock and then the phone into dock. If the phone can see the keyboard then you have the phone plugged in correctly. If the phone can’t see the keyboard then you probably haven’t pushed it down enough. This applies even if there is no power to the dock (provided that the phone’s battery has enough juice to keep the phone running during the testing - around 500 mA - plus enough juice to power the keyboard - maybe 100 mA maximum, plus the dock itself).