So you think that 73 hours before is too soon and 71 hours before is too late? 72 hours before arrival and any time after that is when you complete the DTAC up to the point where you hand over your passport.
Three Visa Exempt entries over 6-7 months might not be be seen as tourist behaviour by your Immigration Officer so be prepared to be either grilled or even denied. You might want to consider the Multi-entry Tourist Visa especially if the Visa Exempt does revert to 30 days any time in the near future.
It is the landlord's notification to Immigration of a foreigner staying. The proof of address is the residence certificate obtained from Immigration which most banks require.
90 days is easy as there a few ways to get a 90 day stay without leaving Thailand (Tourist Visa (and, currently, Visa Exempt), 60 days + 30 days extension, Non-O over 50 (90 days)). Will you be visiting once a year or more because the Non-O over 50 plus the 12 month extension might suit such a plan?