Product is ready and I can establish the firm. However, it seems I should find another way to solve trust issue. I don't know how.... Any ideas?
Well ... This is what I would do if I were in your situation: Given that the excel sheet has everything (according to what you said), if it works on Excel it can be put to work as a web services. Coding such a thing could cost you between 40$K & 150$K. Depending on who you use.
Specifically, one thing is to code Facebook for 500 users, one thing is to code it for 1 billion. Size matters, and because your requirements are a few users, you don't need to worry about scaling the product. Even if you use a lot of data, speed shouldn't be an issue, thus, you don't need to pay for that, keeping the cost low.
So, for around 100$K you should have that service up & running. Might not be fast in calculation, then again you don't need that, and if a customer needs speed, well, then its a chargeable customization to them.
Therefor, you can hire a good programmer, make them sign an NDA and you're covered, you shouldn't worry about that IP.
Last but not least, I just checked your profile after writing all these words: You're here in Montreal! Let me know when we can have a coffee and discuss :-)
Best of lucks!