I know it's old fashioned and out of style with kids these days, but I believe there is real value to PM style shown in the book The Mythical Man Month by Fred Brooks. Here is what Brooks might recommend to you:

1. How do you keep ambiguity at bay?

You have a nailed specification. If there isn't one stop everything and put one into place right away.

2. How do you manage your projects so you don't lose control?

Someone needs to be the Project Manager. By the way Peter Drucker would tell you that it's hard to be the maker and the manager (in fact he advises against it).

So as a PM you have to estimate man hours (vs. calendar time). Break down every feature and figure out how many hours are required. A trick I use as a PM is I ask my team members "Is this something that takes an hour, a day, a week, a month or a year?". I then track their estimated time based on track record and make adjustments (some coders are good at this, some are bad).

If you add or change a feature, revise your estimate.

3. How do you keep motivation of your founding team up when you're slogging?

It depends why it's going south: Is this a death march, or is it just a lack of a plan? It also depends on why the team members are doing the project to start with.