As linked to in #4 the tide stream charts will allow you to work out your timings, the flow in the race ties in nicely with Dover tide tables, in fact the current flow direction in the center section of the English Channel is inline with HW/LW Dover. Dover tables are under the tide tables tab on...
"To lift a mass of 1kg underwater the airbag needs to displace 1kg of water."
What's the weight of the chain and the size of the buoy? You will also have to factor in the force required to break the chain out from the sea bed, another factor is that if the buoy submerges, its lifting ability...
That sounds reasonable, if they want to maintain good will they should be able to sort something out with a sister Co.
To go elsewhere may well involve you requiring a new survey which would be a PITA.
Delivery trip. Whatever fails, buy the tools for the job and charge the owner.
You will never have the right tools or parts for any problems that may occur.
The pic only shows the area that YOU are considering what does the transom area look like? It could be an alternative.
From what is seen: On the side or bottom of the wheel console?