An update -
After much trial and error, I have finally gotten it to work.
I discovered that my formulae needed to be:
For the MSLB rule:
substring(xdMath:Eval(xdMath:Eval(../../my:group1/Value[. != ""], 'concat("; ", .)'), ".."), 2)
(the error I had been receiving when trying to set it to "Value" was that it needed an additional ../ at the beginning, which took lots of trial and error to figure out)
and for the value of the text field:
substring(xdMath:Eval(xdMath:Eval(../my:group1/Value[. != ""], 'concat("; ", .)'), ".."), 2)
Thank you to all who had posted the original solutions to this, it was only by working off of your solutions that I was able to do this! :)