You are over thinking it. The L-pad is going to vary the impedance anyways. I did this on Stern Harley Davidson, but the backbox speakers are wired in series. So I was able to balance the volume level between two series wired backbox and the mono cab speaker, except that I had already replaced the cab speaker with a subwoofer speaker lying around, which the games amp could not drive very well. I added a speaker-to-line level converter and separate 100w amplifier board to the cabinet. Now it's excellent, I can dial in any levels I want.

It will be fine

I had this adapter lying around my shop. Plenty of juice for this application. I spliced in a switched outlet after the game's power switch enclosure to turn it on and off with the game. The amp sells for $15 on Amazon (TPA3116D2 Mono Amplifier)
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I've done external too. The sound quality is somewhat less unless you spend more on a quality subwoofer that is more powerful. You still have to run speaker and power cables, so there's no real gain there.

When mounting internally, the game cabinet becomes the speaker resonating box, so you need less power to give you some vibration that you can feel, especially in your hands. Coupled with shaker motor in the Harley pin, the whole game shakes like a motorcycle - it almost makes you feel like you should climb on and ride it.

I should note that when I have added any kind of audio upgrades my goal is just to improve the sound experience for the player, not to rattle the house windows. That's not my desire, so if you want that it would be a much different path to get there.