7 quarts is too much oil. Some will get thrown out or at least the system won't work proplerly. This could be part of the problem.
Going 15,000 miles on a new car that the mfr recommends 5000 mile changes has put you into a problem. If there ever is any damage and you take it to Lexus they have a perfectly good out from the warranty. I don't see where Lexus is at fault in any of this. The cars are great, but you can't expect them to do everything for you.
I have a 06 IS350 and change with synthetic blend every 3000 miles, simply because I'm old school. I also check the oil level every week or two depending how much I drive. I've never seen any drop in oil level at all. I'm of the opinion that I have to take care of the car if it's going to take care of me.
I'd say you never reached low enough oil pressure to get a warning. I guess what you really need is a low oil level warning light.