Just a shout-out for a GREAT company we've been using for all things diligence and auditing, after more than a few bad experiences with other companies in the space over the years.
FastCTO led by Igor Belagorudsky:
Igor leads a network of CTO's and has a great system in place for providing one-off diligence or ongoing consulting with legitimate CTO's. They can get under the hood for all things technical, evaluate scalability and set up your roadmap for growth (they've done this for us and we're very happy with the results).
Feel free to ping me with any questions or drop my name as a reference if you're trying to get a time-sensitive consult with Igor.