Oops, your #4 is null and void if you do your #6.
Yes indeed the 600LT is an easier car to get in an out than the 675LT but only if you spec comfort seats. The Senna seats make the car much harder to get into and out of than an 675LT with either race buckets or comfort seats. The lip on the Senna seats is quite high and quite thin. It's not as much an issue in the Senna because the Senna has doors that cut out a portion of the roof (like the 720S) so you can rise up and then plant yourself down on the door sills (not the seat sills, they hurt way too much) and you pretty much have to because trying to exit the Senna the way you'd exit a 675LT or P1 with race buckets is just way more awkward (and painful!) even with the roof cut-outs in the Senna. The race buckets from 675LT have a much wider sill on them so you don't hurt yourself trying to rest on them. Plus, where your butt is they're mostly at the same level as the door sills so they form a smoother line out of the car.
The 600LT has a fixed roof so there's no rising up to get out of the car or "falling" down into the seat to get in. It's navigable but your argument that it is easier to get in and out of the 600LT is voided if you get the Senna seats - it's actually much more awkward and uncomfortable. And if you're tall and wearing a race helmet and HANS device, good luck to you! That said, the Senna seats are comfortable and light. If you're only using it as a track car, and you don't find the regular race buckets to your liking, they're certainly one way to go but if you're going to daily drive it you might regret the Senna seats in a 600LT.
In terms of ingress-egress I'd stack rank the seat options as: 1. Heated, electric comfort seats, 2. Manual comfort seats, 3. Traditional McLaren Race Buckets, 4. Senna seats.