Reading between the lines, “until further notice” probably means that it won’t happen for a year or two. Don’t worry, three milliseconds after there are any further announcements from the government, there will be lots of posts here. Until that time, there’s nothing but speculation.
There is no “most airlines” rule. The cost of changing your return ticket varies from airline to airline, and also depends on which specific fare you choose (some fares are more flexible than others).
My first thought while reading your question was in the hills north and north-west from Chiang Rai, which happens to be one of my favourite areas (and for the same reasons as yourself). Another area worth considering is the Mae Kampong district, east from Chiang Mai. The Mae Kampong village sits above 1,000m (if I recall correctly). Good luck.
I see, bugger. Oh well, an emergency passport to repatriate is your only option. A long shot, and I’m assuming you have a scanned copy of your old passport, phone the consulate and ask is a copy sufficient proof?
It’s never a good plan to intentionally overstay. Go to Laos for a few days and re-enter, so you get another 60 days visa free (assuming that is what you qualify for).