I'm sure there's plenty of confusion about the why's and wherefore's of house orientation. The reality, as I have recently discovered, is that if want to live in an area that is older, developed, close to facilities, you take whatever orientation the block actually happens to be and do the best you can with it. Unless you're out in the country, or buying something in a new estate, you may not get a choice of orientation: we certainly didn't. And yes, our garage does have the best aspect - because there was nowhere else to put it - it had to face the street - but we live walking distance to shops (think of the petrol saved on not having to get into the car), we've got clerestory windows on the west side so no afternoon sun issues, and we're assembling a collection of sails for the south side, where the deck is, to provide shade from the late afternoon sun. Oh, and skylights for the kitchen. No, not ideal. But the location is, and the designers have worked around the constraints of orientation. And that's reality for a lot of us.

If you search DIY banquette on Pintrest you’ll see instructions and measurements. Most are in inches but you can convert them. Could you measure the seat height of the chairs you are using with the table (you need the table first) subtract the cushion height and that will give you the bench height. It does depend on the height and weight of those that will be using it the most, to get the best fit.

You have an ensuite and an identical bathroom around 3 steps further apart. Can't see any advantage for this. You could lose the ens, relocate the WIR and end up with a bigger bedroom, bigger WIR and save money. Your roof has these eaves all close together over what appears to be some upper level windows, but these walls aren't reflected below. If I've understood this correctly you've added significant build cost for little gain. I think the room sizes are ok but I would suggest a design professional to polish it up.

for the banquette to be wide enough to lay on it would need to be at least as deep as a narrow single bed which makes it too wide to sit on comfortably without a very thick back rest or a giant pile of cushions to clutter up the space...why not consider a back and sideless freestanding daybed so the primary purpose is laying down and the sitting to dine is only short term and doesn't need a backrest ....this example or similar with taller legs would be a simple economical way to add the day bed feeling without expensive built ins and two along the window wall could look quite inviting with a simple throw and scatter cushion