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I will repeat the thought that you are over-thinking this whole thing. Firstly, you don't draw the run current from the dome circuit, you only draw the signal from the ECU-side ground wire that controls the dim relative to the door trigger inputs. Relays and diodes are there to isolate the circuits, not drive them directly. If you aren't grasping that, speak with an experienced MECP professional.
As for the underglows, you're using older tech and integrating it with new, so you're going to feel wanting when it's done. Your choice obviously, but you will notice the dated look. LEDs are now the preferred option for many reasons, including robust tolerance and cheap manufacture costs.
Do more research into how the door trigger and dome light circuits work in these cars. You have it completely backwards in believing that power is supplied. Power is constant to the dome light at all times, it's the ground that is your trigger.
Secondly, it doesn't matter what route you take the dealership is going to blame it on whatever modification you do. If your fear is having warranty issues with these mods, then don't do them. Again you run a larger risk by creating an entirely new system and then integrating that into the vehicle, don't think they won't notice. Especially considering the extensive work it would take to remove said system in case of dealer repairs. I repeat, cleaner install is the better install.
Many people get remote starts installed on their brand new or leased vehicles, and remove them after their term is up. We don't run a whole new system, we install to the factory system and use all the triggers it provides. This is the exact same concept.
This may offend you, but it's obvious you know just enough about electronics to be dangerous. That is the core of why you're over-analyzing the crap out of this situation. Do what you feel is best for you, but you asked for reasons not to and I gave them along with thorough explanations from practical experience. It's your choice which route you prefer to go.
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