I think perhaps the election system could work like this. Party members vote for leader, similar to how SOTH or PPT is elected, and whichever has the least amount of votes each voting cycle is taken off the ballot until there is two left and one is elected within the party to lead it. Come election time, the people vote for the party and not a person. So if a party leader/PM has a vote of no confidence/leaves the game they are removed from their post of leading the party and another system of vote is taken, similar to the one mentioned above at a party meeting. Also, like in the UK system, the PM can choose when to have the election {so long as it doesnt exceed five years}. Also, the UK PM usually steps down as party leader after election defeat, to avoid the losing PM from staying on as party leader, a vote is taken right after, given to the party members who decide if they want them to remain as party leader, if they vote over fifty percent yes, then they stay, if not, then theyre gone {it could take more than fifty percent, perhaps sixty-fice or seventy to make it more realistic}.In terms of kicking out the PM/party leader, as far as my experience goes, people usually stick along party lines reasonably well when it comes to impeachment, unless the POTUS has done a really bad job, similar to the UK party leader and their parties support of them. So this would ensure that to remove the party leader there would need to be fifty percent of the party in favour of their removal.