Hello !
With the Fiance Visa, you MUST get married within the six month period AND prove to the embassy that a wedding has been arranged. After the six month period (and assuming you are married) you can apply in the UK for "Further leave to Remain (FLR) for two years.
I read about the Marriage Visitor's visa and I THINK the difference between this Visa and the Fiance visa is that the Fiance' visa is classed as a settlement visa (hence the cost) and the marriage visitors visa is like a visitor's visa with a marriage licenece, but on the condition that the visa holder will return to his / her home country, so its non settlement.
In terms of renting your room, you can mention it to the embassy BUT it will only count as secondary income if you can PROVE the source, ie rent book, rental receipts or the funds being paid into your bank account. Also, and this is important, you need to prove to the embassy that your house has the correct "people-to-rooms" ratio, you say its a 3 bedroom, so thats ok, but you must show photos and provide evidence that there is enough room for the three of you in the house. IF there was three of you in a one bedroom house, for example, the visa would more than likely be rejected.
Hope this helps and good luck !
Simon
