Under the current system, you have to do a 90-day report in person at the local immigration office in case of:
- the very first report of the series of initial visa and extensions
- when changed to the new passport
- when travelling back from abroad
Yes, it would be the first 90-day report for your Non-O based on retirement and the following extension(s). After the actual consecutive 90 days in Thailand for the first time is the "first" 90-day report due.
Also, currently, the first 90-day report after travel back from abroad (you left the country and came back) has to be done in person at the local immigration office, even if it's the 2nd or 10th report.
You can try online reporting first anyway. If not approved, you just go to the immigration office to report in person.
Just a reminder. The documents that are accepted at the Thai embassy A might not be accepted or enough at the Thai Embassy B. Processing time also hugely vary by embassy. You need to check the details at the exact Royal Thai Embassy or Royal Thai Consulate-General that you are applying for your visa at/through.
First question - Do I need to inform labour department to cancel my work permit even if I work until the very last day or it automatically expires?
-> As a rule, you have to cancel your work permit officially even if it's expired.
Second Question - Do I have to leave the country once the work permit expires or I can stay until my non immigrant b ends in February?
-> No. You are on an extension based on employment (as you have longer than 90 days already) now and it requires you to have a work permit. Your extension gets invalid without a valid WP.
technically, you will be on overstay if you keep staying on your current extension after WP is cancelled.
Third question - Do I need to cancel my non immigrant b visa by notifying somehow as I plan to apply for a DTV in the future?
-> Yes, you have to cancel your extension based on employment officially when your WP is cancelled. Keeping your WP and/or extension based on employment would conflict with DTV's condition, "employment prohibited".
It's too new to know how everything would go under those though.