Can you get 30 days added (1900 Baht stamp at immigration) after final extension of an Ed visa, or do you have to leave the country 100% at that point of the last day of the 90-day extension?

If yes for further 30 days, can you apply for position on the online immigration queue system for it in Chiang Mai ?

No

All that would be available would be a 1900 Bht 7 day 'extension' during which you must leave the country -------------