I thought we were just supposed to pretend Krige and Thompson were the same Borg Queen physically.
The Queen identified herself as a different species number than humanity. Don't ask me to recall from memory, but I'm sure it's on Memory Alpha.
I'm sure people would be trying to use the different actresses to somehow "justify" this plot arc that thankfully never happened, but that's a weak arguement to say the least. I suppose Ziyal was just getting plastic surgery every few weeks or Cardassians shed their ridges or something too?
I dunno, most of the ideas of the proposed season 5 of Enterprise just made me cringe and think it was best they ended on a high note... well not sucking anyways on Season 4 minus Storm Front and TATV.