Migration assistant questions

The machines presently have the same user name.

How to move your content to a new Mac describes what happens:

"... If you transfer an account that has the same name as an account on your new Mac, you're asked to rename the old account or replace the one on your new Mac. If you rename, the old account appears as a separate user on your new Mac, with a separate home folder and login. If you replace, the old account overwrites the account on your new Mac, including everything in its home folder."

In other words if both your Macs have a User Account named John you will need to reconcile that before Migration Assistant will continue. If you want to keep the existing User Account, then Migration Assistant will create a new User Account named "John (2)" or something like it.

Otherwise, it gets overwritten.

The Wifi will take forever, I reckon.

Not necessarily. It depends on one's particular wireless environment. Migration Assistant involves a lot of "overhead" and takes a long time regardless of the connection method. Wired obviates the vagaries of wireless communication though. It's up to you to determine if it's worth the inconvenience of finding appropriate cables and adapters.

No... and if you must prematurely terminate the migration for whatever reason, that partially migrated User Account is likely to have been corrupted. That's why it's a good idea not to overwrite an existing Account. If you have the disk space, creating a redundant User Account with a name you don't want to keep is the more prudent option. Its name can be corrected later.

It really depends on the homes around you. If there aren't any, great!

Click a Wi-Fi menu. Do networks other than yours appear? Even if the answer is no, networks can be hidden so it's not definitive.

Here is the USB cable adapter if you want to pursue that option: Apple USB-C to USB Adapter