As for the event of any split, you have to ask yourself that although you pay the majority of the mortgage, would you be able to do so if your partner wasn't contributing to other household costs? It has to be seen as an overall picture and the total cost of household expenses and not just the mortgage. For example, I paid for all the bills and mortgage, whilst my wife paid for all the kid clothing and weekly shopping. I wouldn't have been able to over pay on the mortgage had she not done that so I would have been happy to agree 50% .. regardless.
yes it wouldn't be an issue. we split the costs evenly up until this heating replacement which i will pay myself the £8.5k. other than that, we put the exact same into a joint account each month, the person renting the room does their share into that account, and the bills then are paid out.
to be honest, i'd rent out the other room if she wasn't here so i'd be better off but that's a different point for a different day.