1) Yes they sound the same but there are differences in how a ASL monitor and ASLtower sound due to stands used on the ASL. If its sound anchors, the sound would be the same or better on the monitors. If its wooden stands, the towers would be better.
2). Is this room you will have some challenges in that the surfaces on the first reflection points (side walls) are different. There are many solutions but be aware that it’s the differences in surfaces that are the biggest challenge to creating a strong image off axis response (off axis is an ATC strength). Different surfaces reflect different frequencies and reflect them differently in response. In other words, getting off axis reflections to be "the same" as on axis is the way to ideal imaging.
Consider the TV wall; you would need to decrease the triangle size, the couch would need to be closer, but it could turn out better if heavily treated. It may be easier to achieve consistency in this position than that large glass surface on one side and the broken wall on the other. Be willing to try this or at least consider it (I have no idea which one will actually turn out to be better, there are so many variables, but it is important to try different approaches and listen. SO my advice is tell your acoustician to treat the room for both positions). General advice is the further from side walls the better (reducing reflections), but there are SO many other factors.
3) Everything affects sound quality. Consider the 150 a platform to work from and you will hear differences in absolutely everything- preamps, cables, cartridges, turntables, etc. It’s what makes active fun I think, as the transparency is so high that differences in things upstream are easily identifiable. What is the "right" place to start is a really big question that may have more to do with your budget. ATC favors analog, so the SCA2 is pure discrete analog. The newer CDA2 preamp/DAC/CD player could be a place to begin knowing you will move this off to a smaller system later. You will have many other issues that occupy your set up phase which could last a while. But to be fair, a zillion other preamps/DACs would also work. To begin, I would avoid a high color preamp or DAC. GO for transparent and neutral to begin. The handling of inputs properly will be something that matters over time, as it affects you in the operation of the system more than the exact listening to it. As you evolve and the industry evolves and you WILL want other devices in the future. Having more than one input, even if it’s analog, would be handy. I would start with a preamp DAC or Preamp + DAC pairing treating the computer is just one input on the system.
4) There are no changes coming to the 150 in the next year or two. Changes come slowly at ATC as between product development in consumer and pro they have a lot going on at any given time. Subwoofers are the current project, as you can see from the C4 introduction.
Have you considered the existing SE150? This uses the ATC discrete amp package and is the best audio (highest resolution) ATC offers at the moment.
5) Why not fly to ATC in England and audition there?
ATC US importer
(Consumer and Pro)