I empathize with your deep concerns on this. I know why you are concerned and afraid about the loss of your code. While at the same time being excited about the opportunity.
You must also appreciate the risk to an investor of providing money and discovering the code was oversold, or you need significantly more money to complete and get to market.
Do your own due diligence
My experience is that this is an important part of the due diligence process. You should be proud and excited that is has come this far. I assume that you have done or are doing your own due diligence on this team and have throughly qualified them. Based on what you are saying -- they are brokers who are going out to raise the money from someone else. Knowing their record and kicking their tires may increase your confidence.
Create a Black Box Element
Something you might want to consider when you do allow them to review the code. Create a "black box" within the code you provide. Is there an essential component of the code-- a moment of brilliance that you could -- lift out of the code reviewed. When (assuming they catch it) the reviewer point to the black box and says "what's in there" -- you say -- "that's the secret sauce that make this work. And you show that "it" is working, and it could only work if that black box works. But it is your secret until they do become an investor.
Get the result regardless
An independent review of the stability, quality and completeness -- the audit -- of your code is a valuable resource to you moving forward regardless of whether you work with this development team or not. Negotiate to get their resulting report, (branded by them) as a product for you to include in your own business plan and investor recruitment material.
If it comes back "bad" -- which I am sure it wont -- then take it and use it as a plan for raising money. "An independent audit said this is what was needed to complete. This is the cost of doing that. . . . "
Can you plan to go viral?
I am very suspicious of anyone whose plan is "to go viral." As a business developer I experience that the same way a personal financial advisor would experience "win the lottery" as a component of a personal retirement plan. We can all do things that make our product/companies attractive. We can prepare for explosive growth. We can plan and budget for explosive growth. But do we plan to go viral? (Okay, okay-- this is now it's own question. I posted it here. I hope you consider these thoughts in the context of this question.)