Get the required documents from your landlady, house books, ID card etc and register her at her address on the system. Then add your address as her rental property. Then you can upload your passport and register yourself at your address whenever you want. 90 day report is quite easy to do on line after the first one but do the tm30 first. It does not matter how many tm30s have been done while you travel as long as the latest one matches up when you do 90 day report. This is what I do now and it takes about 5 minutes to do both and saves my landlady a lot of grief!
I had no problems changing from an agent to getting my stamp at my local office once I had managed to meet all the criteria. The only thing my local office would not do is transfer my stamp to my new passport. They did not check that I had met the financial requirements for the previous year. I used a different bank account with the banked money which showed it had been there for three months prior to application. That was good enough. I'm not saying that this would be the case at every office.