Regarding the forthcoming Commons vote, what exactly are they voting on, a commitment/aspiration, or something placed in law? I believe the change to fixed length parliaments of 5 years was effectively placed into law, so the next government has to honour it or remove it from the statue books.
If 2% GDP on defence is a commitment/aspiration, then the next government can ignore it, if it is somehow enshrined in law, then the situation is as above, the next government will have to honour or remove from law. If the next government has an overall majority they can pretty much do whatever they want eventually.
That is my (admittedly limited) understanding of the situation, I'm sure someone will be along shortly who is better informed and will enlighten us all!