1) Embassy voting is not a real thing. Nor would it make any sense, as different states handle elections differently, but embassies are a unified federal system. Nor does the US have embassies in every country. Nor are embassies guaranteed to be anywhere remotely near where a person lives within a country.
2) "Advance voting" makes no sense for expats. Believe it or not, some citizens live overseas. Including the military, by the way, who you apparently want to disenfranchise.
3) Your #1 case does nothing to guarantee vote privacy. The person can very well watch the individual fill out their ballot and then seal it up. All it does is make it harder/more expensive for the disabled to vote.