Superb job John! =D>
And it's brilliant to see someone tackling a chair on the forum, I take my hat off to you.
It's probably sensible to use an upholsterer for the seats, I'm happy to pay them to do the job because they're better at it than I am. However, when it comes to drop-in seating like this, then I do provide them with a 12 or 15mm ply insert that's individually fitted for each seat. The reason being that's a job I can do better than the upholsterer.
Take the thickness of the seat covering material with some callipers, add 0.5mm, then leave this gap all around. I normally start with a tightly scribed ply insert then pencil in and remove the required relief. Personally I prefer to just put a rebate in the side and front rails (I only rebate the back rail if it's curved), but there are simpler ways of doing the job.
I can't see clearly from the photo, but the corner blocks should be scribed, glued and screwed, with the screws
perpendicular to the rails they're being screwed into. Again it's a personal thing, but I prefer to support the drop in seat on the rebate rather than the corner blocks, so position my corner blocks 0.5-1.0mm below the bottom of the rebate.
I think you've done absolutely the right thing keeping the weight down, once a chair goes over about 5Kg it becomes increasingly awkward to use. Okay if you're a beefy young bloke or you have loads of servants, but with a heavier chair your elderly grandmother would need help every time she wants to sit or stand.
I hope you've been bitten with the chair making bug and decide to try something more challenging, if so don't hesitate to shout if you need any help! Once again, congratulations on a first class job.