Multi entry tourist visa is 6 months 60 days at a time although correct timing can get nearly 9 months out of it.
Dont think there are multi entry O visas now available unfortunately. There is the OA visa which is valid for a year bit has insurance and stuff like that.
OA entry can be for whole year or as long as your insurance is valid i have known people get nearly 2 years with this visa
If so need to go british embassy to make an affirmation for marriage i believe its called. This was the longest step for us.
We used a company near British embassy at that time (express translations) to get what we needed translated. Got them to go to MFA as well to get it all certified.
there is no 99 year lease, some projects were promising 30 + 30 + 30 but recent court case showed only the 30 year lease was legal and its entirely up to the owner if they want to renew