How can I secure my idea at the beginning of the process vs my subcontractor engineer whom I hire to implement my idea?
And the same question regarding all possible manufacturers that I need to contact to find out the prices to check the profitability of my potential product?
This is usually accomplished using a non disclosure agreement.
However, you also need to consider that the engineer might make some contributions that amount to making them a co-inventor. You will want an agreement that has them assign any rights they may legally have to you or your company. Such an agreement does not change who actually invented what but it gives you the rights that initially accrue to them as an inventor within the scope of your project.
All inventors need to be named in the patent application.
From one of your comments:
I want somebody to create 3D photorealistic images of the product, before I start application for the patent; if the guy steals my idea or passes it to his friends/relatives (who might be able to commercialize the idea faster than me) there will be no impact on his reputation. What should I do in such a case? Will I be able to prove later on during patent filing that I was the real author of the idea? How can I prove that I and not his friend X was the author of the idea?
You really should consider filing a provisional patent application. These are generally easier to file and will provide one year of protection to allow you to draft and apply for a non-provisional application. This will provide the proof you need to show you are the inventor.
That said, you could consider contracting with an actual product development firm. They would have a lot to lose if they violated confidentiality. This would probably be somewhat more expensive than a solo contractor but I've never heard of a product development firm stealing inventions, ever.
