All the good things about old acquaintances should never be forgotten; the bad stuff, never brought to mind.
'Should old acquaintance be forgot'? Does that mean that we should forget old acquaintances, or does it mean if we happened to forget them, we should remember them, which is not possible because we already forgot?"
Of course they shouldn't! That's why we love the song so much. It makes it very clear how important it is to remember our old friends.
I can't believe you opened the champagne without telling me! And it sounds like you've finished the entire bottle all by yourself...
I will remember Mrs kelly when I leave
Just couldn't wait to drink that champagne, could you?
That last bit should be 'for the sake of auld lang syne'.