1) They allowed non-residents to apply before e-visa so likely still allow it.
2) Both Cambodia and Vietnam have switched to e-visa so everything is done online once you arrive in the country. You don't use the embassy website, you use the e-visa website.
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3) You can't submit anything through someone else. You apply yourself and upload your documents online directly to the evisa system.
4) Check the embassy website for their requirements. It should say what language documents must be in.