Let's clarify the options:
loop_request: determines the type of LOOP packets to get. If you set it to 3, you will get a mix of legacy type 1, and the newer type 2, LOOP packets. This is covered in the [Vantage] section
of the User's Guide.
archive_interval: as you say, for stations with hardware-specified archive intervals, such as the Davis stations, this is ignored. Instead, WeeWX will use the value in your hardware (which seems to be 1 minute). WeeWX then uses this value to determine when to stop requesting LOOP packets and, instead, process archive records. If a station can generate archive records in hardware, as your Davis Vue can, then those records will be taken from the hardware, then processed and stored in the database.
To answer your questions:
1. Yes. So, in your case, every 60 seconds WeeWX will stop requesting LOOP packets then request any new archive records from your logger.
2. loop_request has nothing to do with the archive records stored in the logger. Changing it is useful because type 2 packets include some information that is not available in either type 1 packets, nor the archive record. Let's take an example: observation type "altimeter", It does not appear in type 1 LOOP packets, nor in archive records. If you set loop_request=1, then WeeWX will have to calculate it in software, then put it in the accumulators. If you set loop_request=2, then WeeWX does not have to calculate it and, instead, simply puts the value that was in the LOOP packet into the accumulators. Either way, at the end of an archive interval, a value is extracted from the accumulator and put in the archive record. With loop_request=1, the value will be from pure software, with loop_request=2, it will be from the Davis hardware.
3. It's true, so this question is moot. However, I will add that the frequency of LOOP packets is hardwired in the Davis stations at 2.5 seconds. It cannot be changed.
Hope this helps.