AbigailII: You are absolutely right.
But let's start from the very beginning. :)

1st paragraph: Because the game would end in 1-2 turns, and as you wrote before, become very uninteresting. So I add another rule, hmmm, I don't know how to put it in english, so here goes:
Current declaration: "2 ones"
possible responces: "2 twos", "2 threes", "2 fours", "2 fives", "2 sixes" and "3 ones"
No higher bids. Clear enough?

2nd paragraph: A matter of tactics. I believe, that at this stage of game, B should declare "2 ones".

3rd paragraph: I missed out one word. B: "LIAR!" B knows A hasn't played fives before, so PROBABLY/ALMOST SURELY he doesn't have any.
And again, A was given 2,2,4,4,6 hand
Would you play "6 fives" having no fives on your hand, hoping opponent has 4 fives? I don't think so.