The only real issue to this approach is whether or not the lighter is live when not in accessory.
So, I might modify the adpater slightly:
1) fuse or PTC fuse
2) two clips
3) A Schottky diode (lower voltage drop). Wired the same way. Band of diode to red clip.
As for a suggested connection point, the alternator battery terminal and ground.
You will have to be only slightly more careful with the + battery cable. Just put it in a zip lock when you remove it.
So, modified removal instructions:
* Verify 9V battery is good (Could add a button/LED test)
* attach clip to + of alternator (usually easy to get to)
* attach clip to ground.
Remove Negative terminal of battery first.
Remove positive terminal. Any accidental ground connections. i.e. ratchet hitting ground could _______compromise backup.
Put positive terminal in zip lock.
Install battery. Positive terminal first. Same precautions.
Negative terminal. Always do this connection quick.
Remove clip from alternator.
Remove clip from frame.
There are blade fuse adapters that you can use too that can give you access to +12 on the batt side. e.g.
https://www.ledcom.co.uk/add_a_circuit_atm_mini_blade_fuse_holder_connector
How many times do you replace a battery? Every 6 years? I recorded by Radio Presets. Now that might not even be an issue on some cars.
Downside of not backing uo: ECM has to relearn AND you may not pass inspection after a battery replacement until after the full test sequence completes. One part is cruising speed for some length of time and at least one warmup/cool down cycle. Codes will be erased. BUT, if you need to replace a battery, it's LIKELY that memory loss may have already occurred.
If it occurs in the driveway and you have another vehicle, you might have a fighting chance. If it occurs 30 miles out of town and you don;t have tools all bets are off. Your insurance company may give you a free tow.