Can I Apply for a Non-O Visa While on Amnesty in Thailand?

You are indeed covered by the blanket amnesty and will be allowed to stay no longer than the 31st of July (watch for the notice when they get the borders open as far as what amnesty holders need to do)

BUT

will you be able to apply for a Non-O visa in country FROM an amnesty stamp? I would say no you cannot. I have had several people denied applications of their in country Non-O visa at Chaengwattana in Bangkok because they didn't have the required time in "real days" on an "hard extension stamp" (that wasn't an unable to travel due to covid stamp OR the no stamp amnesty)

And did you use the embassy letter to get first an under consideration stamp for 1900baht and then go back and get the second 'extension of stay" stamp after that for free?