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.
Not sure what you’re meaning by a single entry visa.
The way the OA works is that the visa is valid for one year on approval.
Whenever you enter Thailand within that first year you are given a stamp for one year or until your medical insurance is valid until whatever is the shorter. You can come and go during this first year as many times as you like.
If you were to leave and return just before th visa itself expires and you have updated medical insurance then when you arrive you’ll be given another year stamp (or until you insurance is valid), effectively making the visa usable for two years.
The second year isn’t multiple entry and as the visa itself has expired if you leave you’d need to buy a re-entry permit (single or multiple) to keep that year stamp valid.