Bit sly of the customer to tell you that after accepting your quote. If that information was upfront it may have saved you time working out a quote if you'd decided it was too much hassle for you.
Try to isolate the lighting circuit inside the garden office and see if house resets ok. If it does then fault is on lighting circuit. If not then isolate sockets and see if it resets. That will at least help electrician narrow fault. It could also be a fault on swa cable, I've seen them...
A compression male coupler would work. Bit of 15mm pipe in compression end as long as those hoses can go over the 15mm pipe, then jubilee them on. If they won't go over the 15mm pipe then a suitable 15mm to 10mm reducer on pipe can be used to reduce down...
There's only 1 inlet there and it's the one with the ballcock. It's probably failed, not uncommon. Isolate mains and change it, then go from there
The other 2 pipes look like overflows.
C24 is a better product, less defects and hopefully less chance of warping.
I'd set rafters at 600 centres if roof is lightweight, you don't mention what you're using as the roof.
How are you attaching rafters to wall mounted timber?
3 X 2 is sufficient with 18 wbp. Also helps keep the tray lower. Take into account height of your trap when setting height and run of waste pipe. Easier to build a solid base around the pipe run than cut afterwards and potentially weaken the base.