Vietnamese Immigration is very chilled compared to the Thai side. I went to Hanoi with only one way flight and 3 nights hotel booking. The IO at the airport didn't ask for anything and stamped my passport.
I only booked a return flight after I got the DTV about seven days later.
So Option A is very feasible, you can also travel around Vietnam if you have time and hop back on the plane from the nearest airport.
Not a problem at all cause I've done it several times with TM-30s that are months behind my real entry.
To give you an extreme example, one time I used a TM 30 that has a blank check out date and the IO asked me to fix it. So I called my agent (our condo has in house agents) who got me a new one the same day (for a fee cause I wanted it asap) . And guess what, the check in date of the new TM 30 was the present date (cause the agent just submitted it) but this one now had the check out date. I went back to the same IO and she didn't even care that I just got it. She's happy to see the check out date and approved.
If the owners are willing to do it, you should ask them. There is no problem with a late submission (even though the legal window is 48 hours or something) cause that's what I do all the time. I always get a fresh TM30 from my condo agent whenever I need to do the 90 days, even though I have checked back into my room months ago.
You can use any address you like. It's not like the embassy staff look for every address on the map. For soft power, the main things they will check are the legitimacy of your gym and your financial statements.
In addition to the arrival stamp, you need to submit at least a hotel booking to prove your location. So you need to apply to the embassy that serves the location you are showing as proof. They sometimes ask for more info like a selfie with the hotel's name or a boarding pass or a screenshot of your google maps.
As for finding out which place falls under which embassy's jurisdiction, as soon as you choose Vietnam in the system, you will see a pop up window with the names of city and places (in red) which fall under which embassy. Da Nang for e.g, falls under HCMC jurisdiction.
Yeah, I think the IO felt that she needed to ask something. I was giving her everything I've got cause my passport pages are not very "clean" 😅. Next time I'll be more confident.