It’s to report 90 consecutive days in Thailand. If you leave prior to that the requirement to report ceases when you leave. The “clock” starts again at day one when you enter.
There is no hard and fast rule about how many days you’re allowed in Thailand per year on Visa exemption entries nor how many per year you can make.
Entry is determined by the immigration officer you hand your passport to. If they deem your entry history seems suspicious (trying to live here or work here) on free entry stamps then they will either tell you that on your next entry you’ll need an actual visa or just flat out deny entry.
No one here can say what will happen to you. It’s up to that individual to decide.
Rarely would shipping furniture and other goods work out cost effective. Apart from the exorbitant shipping costs you’re likely to pay a good amount of import duty and taxes. A figure the customs guys will pluck out of the ether based on your bill of lading. If you question it they’ll go through the shipment with a fine tooth comb and invariably you’ll have a higher assessment.
Not true. In theory anyone can do a 90 day report for anyone else. Will be dependent on the office they use as to whether they will accept that though.