Yes....three flooring types inside of 2 feet is two too many. The issue becomes which floor to keep and which two to get rid of. Right now you have the maximum amount of flooring options in a single space = 2. If you add a 3rd you will be kicking yourself in 1-2 years. It looks like tile is the best option for your situation so why not go with tile? I'm guessing the answer is, "Because there is a lot of tile in the home and I want to keep this a small project." If you want to replace the carpet on the stairs and the sitting room, you are probably better off replacing with more carpet. This will allow for a refresh on a small scale. You will do this knowing full well that a massive flooring update will be coming in the next 5-7 years. It will give you time to save up for all new flooring.

I don't think the assessment that "it won't grow grass" is accurate. I say this because on the lilac side and the fence side of the mud strip, and all around elsewhere, there is grass growing. It would be more fair to say you're not doing the things required in order to grow grass in that particular spot. That said, if you wanted to grow additional plants, you'd need to create a bed for them to which grass is excluded. At the least, a bed should be created for the lilac, as no grass should be growing among its canes. IMO, the space is not begging for something to be added in order for the area to look better. What is begging is that poor lilac, which I really couldn't call a tree at this point. It's more like a scruffy, bedraggled bush. In order for it to become something someone would want to look at, it needs to be refined into a reasonably symmetrical and tidy shape. Here's a guideline that might help. I'm suggesting removing some top and side growth to help the canopy thicken up. After a shape is underway, you can allow it to grow a foot or so higher (whatever it is willing to do without scraggliness) every year. Take a look at nearby trees and see if they don't need some of their lower limbs removed, which could help considerably in allowing more light to get to the lawn or other lower growing plants.

Regarding your question about laying something over the marble, in our last home the previous owners laid ceramic over ceramic and we lived there for over 25 years - no cracks. It will be a pain though for anyone down the road who is updating, but you could ask a pro to be sure (will need a transition piece at doorways). The area where closet was will have to be filled in and very level. If it were me, I’d bite the bullet and remove the marble and try to match up to your wood floor or use a more casual, rustic feel. You may look back and wish you had, especially if the marble looks patched.

I love everything you chose! I think your issue is actually arrangement. Instead of centering the bench, push it all the way to the right to where the plant sits currently. I'd keep your existing mirror and add several more in a grouping over the bench, a small gallery wall of mirrors. You have natural asymmetry because of the switchplate, location so roll with it rather than fight it. Dip some of the mirrors down closer to the bench. Mix wood and gold-tone frames. You'll need to relo the coat hooks...that's ok. I'd actually stagger 5 closer to the door where there's a top row of three and a bottom row of two in the gaps from the top. Throw a small mirror or two over on that side as well. Depending on your ceiling height, and if you still have the parts for the chandelier, trade out the drop piece you used for the next longer  one to bring it a bit lower. From there, just add some more greenery and a few more pillows.