you won't need to report if you're not here for the full 90 days. If you're on an active Non-OA visa you can just exit and return. However if you're Non-OA has expired or (more likely) you're actually on an extension of stay based on a Non-O visa make sure you have a re-entry permit before you exit
please reread what I wrote above. I don't care how big your company is, the reason you had to go to Singapore for your Non-B is that your company was either unwilling or unable to supply the much greater amount of paperwork required to get a Non-B visa inside Thailand. That doesn't mean you can't get one inside Thailand which you can do if you meet the criteria.
I'm afraid not. It's been the law for a very long time. There was an official announcement last year that changes how the Thai Revenue Department treats income brought into Thailand for tax purposes. It came into effect on the 1st of January this year.
like I said it depends if they are eligible to apply. That includes having the correct paperwork to apply. The paperwork supplied by the company can be pretty extensive to apply inside Thailand so many prefer to supply the reduced paperwork required to apply outside Thailand