Please take a step back and try to relax. Reception teachers are well use to wriggly children and no decent teacher is going to equate wriggly with naughty. Anyone who does clearly doesn't understand child development.
It's normal for children to still struggle with fiddly fastenings at the start of Reception, but once he's doing it daily he'll soon get there. The best thing you can do is encourage him to be independent in putting things like his shoes and coat on, as this will help him when he starts. Staff will always help a child who is stuck but they'll expect them to try first. It's always obvious when they start who is use to their parents doing absolutely everything for them, as they will simply hand everything to the adults expecting them to do it for them. As long as he is use to trying, he will get there with practise. It sounds like you've made sensible choices with footwear etc, you wouldn't believe how many buy laces for a Reception child! Getting him to wear bits around the house is a good idea if you think he needs time to get use to it all, but not essential.
His concentration should naturally improve as he gets older, and school often helps with this as it's more structured than nursery. They are only expected to sit for 5 minutes or so at a time to start with, but this will be for several carpet sessions a day so those who struggle with it initially get use to it quite quickly. Most children generally accept carpet time as part of the school routine. Obviously if he doesn't get use to it and still can't sit for more than a couple of minutes by Christmas that could be a sign of an issue, but you and his school will tackle that together if and when the time comes. For now, they won't think anything of it if he struggles to sit for more than a few minutes. He won't be the only one either.
It depends what you mean by encouraging him to sit at the table, but I'd definitely encourage this for meal times if he doesn't do so already, as that will be an expectation at school that he sits at the table to eat lunch (again, this is something we are finding several children struggle with each year initially, if they're not use to structured meal times at home). Sitting to eat lunch and being able to feed himself independently are all part of school readiness, so if he does those things already that's great.
It really is all about independence when it comes to school readiness so please try not to worry about things like focus and concentration, or anything academic. That will all come in time.
If he can separate from you reasonably ok (as in settles after a few minutes, even if initially upset), can attempt to dress himself independently, can feed himself and manage toilet needs reasonably independently, and communicate his basic needs effectively etc, then he's more than ready to start school. You haven't messed anything up at all. School will expect the odd tears at drop off (especially initially), they'll expect him to still need help with the odd item of clothing, or still need occasional assistance with toileting etc.