One way to tackle #1 (I'm sure there are other probably better ways) would be to use the framing and mosaic wizard as you've done and set the sequence to take more than one 10 min exposure. Once the first image is downloaded, pause the sequence and look at the image statistics and see what the "ideal exp time" is. While paused change the events to suit your needs.
As for #2 you can add, remove or change events on the fly. Likewise you can add or remove targets to/from a sequence while it is running. If you have an internet connection you can use the framing and mosaic wizard to "append" a target to the current sequence.
Others are much more proficient than I am and can give you more info, but the above should work. One great thing about SGP is its flexibility.
HTH
Mark