if you suffer from the pollution you could just find a place a bit outside of HCMC and check your e-mail regularly. They won't ask you for an interview the next hour... more likely the next day or so.
yes you can both apply and stay in a different place than Ho Chi Minh City (when you apply you need to be in the south of Vietnam/the area of Vietnam where the HCMC consulate has jurisdiction). You just have to prepare to go to HCMC for an interview IF they request an interview. It is rare, but they reserve the right to ask for one if they think it is needed.
I suggest that you make an account on the e-visa platform and start checking out what information you are supposed to provide...
That being said,
- no you don't send the passport. IF your application is approved you will get an e-mail with a pdf-file to print out (to present together with your passport).
- no, you don't have to travel immediatly, you have 5 year to enter.
- on the e-visa platform you are asked for your travel plans... You are supposed to enter your intended date of arrival, a flight number for tickets you plan to buy, details for accomodation where you plan to stay etc. There have been reports in this group, where the embassy request airline tickets and proof of accomodation as additional information.
Use the search tool in this group and look for information for the specific embassy you plan to apply through. Also read the information they provide on their web page.
well sure - people have done it without issues... That does not mean it is recommended or that you are supposed to do so!
Embassies clearly state that they can request an interview (reports show that they can call you in the next day) and there are loads of examples where applicants have been asked to submit additional documents including copies of all passport pages to check that you are still in the country.
What about this do you not understand?! Why do you keep questioning the advise to stay in the country of application until the visa is processed?!?