No it doesn't. Parties in parliamentary forms of gov't form coalitions all the time. In fact, this is the reason why the Tories are currently in power and David Cameron is Prime Minister, with the Tories having formed a coalition with the Liberal Democrats. If the Tories got 43% and UKIP got 12%, they would almost certainly form a coalition. And by the way, you have left the Liberal Democrats out of your calculations.
Yes,they do.However I have seen one blessed suggested that UKIP will win any more than a few seats in a General Election.OTOH,they could easily take enough votes from Conservatives in a marginal constituency to give Labour the seat with 35% or 40% of the vote.
Remember....BillyBobBlythe 43%...Ross Perot 20%.
If in an election for a particular seat, Labour gets 40% of the vote, the Tories get 38%, and UKIP gets 22%, then Labour gets the seat. Multiply that by dozens of races where UKIP and the Tories split the conservative vote among them, and you have Labour getting a majority of seats even though they lose the national popular vote.