You can ALWAYS be asked for proof of onward travel when entering visa exempt. It's a requirement for that entry type. Whether it's your first entry or your 10th.
It's not common to be asked by immigration, unless they are looking for a reason to cause trouble for you.
2nd entry in 12 months isn't much information. That could mean your last entry was a year ago, or 3 months ago. Those are not the same for your chances.
It's much more common for your airline to require this as part of your check-in though.
There's no such thing as a 1 year non-O. Non-O visas are all 90 days.
If you want more time than that, you go to your immigration office and ask them for their list of requirements to apply for the 1-year extension. Then you apply for the extension based on their list of requirements. Then you can purchase a re-entry permit for the extension. You would have to extend every year. Likely by showing 800,000 baht a bank account in Thailand.
If your immigration office supports in-country conversions to ED visa then you can just leave and return and go to immigration with the proper paperwork.
Just be warned there are many reports of some Bangkok Bank branches refusing to provide the letter for monthly transfers. I've seen most people resolve that by going to a different branch and getting it there.