BTW can the rear-facing seats be retrofitted?
No, the rear facing seats mean that the entire rear frame of the S is different. It has to be reinforced so that the occupants are sitting in the passenger compartment section of the frame and not in what would otherwise be the rear crumple zone. This is why it's a factory option only, the aluminium and steel frame at the rear of the car is completely different with the 3rd row in the S.
The S model is the only car that seems to bridge the dilemma of not having a huge car but also accommodate everyone. When the kids grow out (currently aged 7,4 and 1) we will either look at other options or remove the seats.
Because of the above, the seats cannot be removed later.
Also, check the height/weight requirements. You may find that your 7yo is already too big, and your 1yo is almost certainly too small at the moment!
From personal experience, putting a baby (2yo and newborn) in the back of an X is incredibly easy. No bending or twisting, no bumped heads (me or kids) etc.
As for size. Although the X is bigger, it isn't bigger my much. The S is still a very wide car by UK standards. The X is only 3.2 inches wider, and 2.3 inches longer than the S. It looks bigger, but actually, there's not much in it. In either car, you're going to worry when you inadvertently end up at a 7-foot width restriction and forget about a 6'6" - nerves of steel required here in either model!
To put this in perspective, the S is less than 1 inch narrower than your Q7, and both the S and X are shorter than the Audi (the X only by half an inch).
As you've said, the look of the car is a personal thing. The S is definitely the better-looking car, but I think the X is great looking too!
And if it actually meets your needs better, in the long run, it would be worth considering. You will find on here plenty of S owners who switched to an X and have said they prefer it.
I think either way if you've never owned a Tesla before you're going to be blown away by it for the first year. It's how it lives with you after that period that is going to make or break your relationship with the car. If you're at all on the fence, borrow each for a weekend and see what they're like to live with. We did this and that's what convinced us to buy the X over the S.
Good luck with your choice!