No, test points are different on each board. The label for TP1 on one board can be completely different than TP1 on another board. You will have to look at the schematics to determine what the expected voltage is supposed to be for each test point on each board.

The difference with/without a load can be as little as 0.01v or as much as 3v (especially when the expected voltage starts getting up to the 50v range or more). It also depends whether or not the voltage is regulated or unregulated. Unregulated voltage tends to fluctuate a lot more.

As long as the voltage is fairly close to the expected voltage without a load (and without going over the expected voltage more than a few mV), it's usually ok to connect the other board(s).

The capacitors on the driver board will almost certainly need to be replaced if they still have the original ones.