There is a lot of stuff in the post and the comments about Lean Startup, but I want to jump in on this the wish expressed 'The Jason Fried Fantasy'. Specifically this:
We would combine simple, opinionated software with crisp software copywriting and a low-touch online sales model.
This limits your options. I think the product was too niche to be a 37signals 'sell to the fortune 5 million' product. I'm passionate about 37signals, but it might be difficult to apply to niche markets that require aggresive sales / long deal flows.
Now you also point out:
For every customer or prospect we talked with, the risks of innovation (failure and losing their job) far outweigh the hypothetical benefits we proclaimed. Customers just want reasonably priced software that does its job, not helping you launch your Lean Startup adventure.
Now assume you've already validated the problem. Probably you were getting the wrong people in your sales funnel. Basically if people fear to lose the job, you need to move up in the companies hierarchy. Look for the manager above him and keep going. Or start at the top and work down if possible. But keep talking until someone keeps calling you back to have their problem solved. Steve Blanks Customer Development talks about this as well.
About the quality of the MVP. It's better to ship a half product than a full featured half working product. From what I read in your post a lot of people weren't feeling safe to introduce this in the organization because it was an immature product. You solve this by narrowing down the featureset or display 'upcoming features' and involve people in developing further. But this is different per market, B2B (as mentioned in another comment) is different than consumers.
Edit: fixed markup.