Can I convert an Ed visa extension to a Non-B visa and then to a marriage visa in Chiang Mai?

Nope, can't go from an ED visa extension to Non-B anything.

You'd have to cancel your ED extension, exit the country and buy a 90 day Non-B at a thai consulate in a nearby country OR (if your employer can meet the in country requirements) bounce out/back to get a free stamp then apply for a 90 day Non-B at your immigration office.

And the other scenario

IF you have a Non-B visa/extension you can change it to an extension based on marriage