Shrug, Smile, and say
"I'm happy to look over your NDA. This is what I do. Just like I would never expect you to provide me services for free, I expect to be compensated when my services are provided."
Then move on if they are still argumentative.
--- and that'll be $3 for looking over your question :) ----
When you hire someone for a task, this establishes a contract. Even just orally. All that matters for both parties is that there is evidence of a mutual agreement.
But it is much better for both parties to have a formal contract, which mostly specifies in advance the scope of the work, and what to do if things go wrong.
You certainly can.... and the Freelancer may have a contract already.
Contracts are designed to clearly identify what is to be done by each party so there are no misunderstandings. It is never a bad idea to write responsibilities down so everyone has the same expectations.
You could contact a french "portage" company and offer services as an adviser to implement and use your software. It sticks to your criterion and will permit you to start monetize your app.
Could you tell us a bit more about your business model (pay as you go ? subscription ?).
You could also start a limited or a partnership (and become a silent partner) ...
You can't write off business expenses before the business entity is formed.
Think logically about what you are asking..... If it were possible to write off education as a business expense for a business which is formed well after the education... every business owner the the US would be writing off any college tuition, even if it took place 20 years prior ...
I'd charge my day rate as a base.
You can figure day rate, based upon desired annual salary here
If work is expected to go into late hours of the day, and extend well past 8hrs per day, then I'd double my day rate.. 16hrs per day. Then multiply by total days of work. So for a7 days... day rate x 14
Granted that only leaves 8hr to sleep then start working ...
This review is for your benefit, not theirs. You are not providing a service for them here, you are managing your own business. To some extent, they don't care how well you review it as long as you sign it. The exposure is yours so why should they pay for it.
Costs like this should be accounted for in your operating budget and feed into your rates, but ...