Not enough info to say for sure.
How many circuits are there?
How far away from the CU is the incoming gas and water?
How far away is the cooker point from CU?
Who will be plastering up chases?
Matter of opinion. I've done loads of T+G floors and they are solid as a rock. Glued joints and glued to joists, no way it's ever going to squeak. Wouldn't catch me doing it your method in a month of sundays.
Green - horizontal continuing what to the left. (grout lines matching)
Red - personally I'd go for horizontal again with a mitred corner continuing across in front of the window.
Wouldn't a better option be to window board the sill reveal?
http://www.snickersdirect.co.uk/snickers-trousers/snickers-workwear-holster-trousers/snickers-trousers-3212-duratwill-work-trousers
I swear by these. Light enough not to sweat you bag off in summer and strong enough not to tear at every given opportunity. Comfortable too.
You will need to take about another 4 inch off what you already have. I'd be tempted tell you to get a digger driver in on a micro digger unless you fancy some long hard evenings. Typical build up will be 4 inch MOT type 1 (compacted) and then a sharp sand and cement mix as bedding material...
Where did you buy your tape from? Are you sure it is self adhesive? May just be a dud roll. Scrim isn't super sticky anyway, a flatten with a trowel after laying it on helps.
Square edge ply? Did u route a tongue and groove onto it? Butting square edges up is a recipe for squeaky floors.
22mm P5 T+G chipboard glued and screwed is the way to go.