You only need. TM30 is you’re applying for an extension. If you leave Thailand before your visa ends or your visa-exempt stamp expires, you don’t need a TM30.
Not true. The traveler can be fined for something the landlord was supposed to do. That’s why it’s advisable to get a copy of your TM30 before you go to Immigration.
I think a 6-month lease will raise more flags than it’s worth. Tourists don’t sign 6-month leases. Someone circumventing the long-term visa system would.
You can buy an onward ticket from onwardticket.com for around $15US or one of their competitors.
I arrived visa-exempt in late October and was asked for an onward ticket by Immigration at Phuket Airport. I understand it’s very rare but it happened to me. Fortunately, I had one; they didn’t ask for it in San Francisco.