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You could have problems with VOA on Cambodian border. But making visa online prevent any issues. Also please check vacine\certificate obligations in every counrty on your way.
As far as I know there are no any other issues with your plans. Just a couple recommendations.
Traveling by bus everywhere will be cheaper rather than by air. And travel on a cheap ...
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Would these stamps always be given on request (e.g., for Americans who want to prove that they did not overstay in a foreign country)?
No, according to https://stuff.mit.edu/people/stransky/us_entry_passport_stamps.html (mirror):
Ports of entry I've used with no stamp received, even after asking:
Laurier, Washington
San Ysidro, California
El Paso, Texas
...
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