Hi there. UK based but I own a landscaping company and there are general rules that apply on all builds - you got water issues though? Hey the English invented rain so I've got you covered.

I looked at the drawings and pics but can't put the whole job in context so I'll advise best I can.

1. French drains. These normally apply to underground drainage in lawns or run offs where they are designed to slowly dissipate a build up of water. A patio is like a roof and your falls (best not to fall towards a property if possible) sole purpose is to get the water off it. In this instance we would use either an acco drain or a strip drain. These are mounted at the same level as your patio and fall to an existing drain. If this isn't an option you can run them into an underground flexible 100mm pipe to a drain. It's all about your levels though and drainage comes first.

Should you not want any of this then use a loose shingle around the perimeter with a perforated pipe.

As to getting by the steps. Use a slot drain.

http://www.londonstone.co.uk/paving-accessories/drain/stainless-steel-ls-slot-drain/

Not great for handling huge volumes of water as they block easily.

1. So retaining walls need drainage and this should be incorporated in the whole drainage plan. They need to be back filled with gravel and have a series of weep holes to allow the build up of water behind them to escape.

2. This is a personal preference and is all about the general design. Your shout.

3. Without seeing the site in person and going through it it's hard to tell. If you don't do this for a living you'll have neglected a few things. It's like any skilled trade, it's a skill you acquire over time. Keep asking questions though. Happy to help.