We are a Digital Agency about to hire a technical partner to work on sweat equity. How do I negotiate the terms?

I developed and published mobile apps as an individual for several years, and only formed a corporation later as things grew and it made sense. As far as Apple's App Store and Google Play are concerned, you can register as an individual developer without having a corporation. I'd be happy to help further over a call if you have any additional questions. Best of luck with your mobile app!

On paper you have the advantage but after several startups control resides in he who knows how to execute the vision of the company.

I'll keep try to keep this answer brief, but there are several factors and nuances that can be discussed in more depth. Where you decide to incorporate partly depends on what your future goals are with your company. Companies that plan to seek venture capital or go public typically choose Delaware as the state of incorporation, and usually choose a C-Corp. Delaware has a very well developed body of law surrounding corporate governance and that provides comfort and more certainty to future VC investors. If you're not planning to seek VC money any time soon, an LLC is a smart decision because of the tax benefits it can provide to you as the owner. It sounds like you want to grow your company on your own without outside financing. If that's the case, I would recommend forming your LLC in California. Regarding California vs. Delaware, one benefit to forming your LLC in California is that you can avoid paying a registered agent fee which can cost anywhere from $100-200 a year. If you plan to seek venture capital down the road, you can reincorporate in Delaware.

You should retain a lawyer, or another qualified individual, to act as the third party designee for the corporation (if you do not have a partner or co-owner who is a US citizen). The designee should prepare Form SS-4 (Application for Employer Identification Number) and Form 8821 (Tax Information Authorization) for the corporation’s president to sign and return.

This sounds as a deja vu to me. I have been in a similar situation back in 2000, we could only solve the issue thanks to a good mediator. However every situation is different and hence your route to a solution might be different. It also depends where you are in the world that defines how an email and/or verbal agreement might be a sufficient ground for legal actions. I am not a lawyer and can not judge that.