Viability is a KPI and not a set of requirements for some product.
You can't plan for a product to be viable. You first build a product and then you verify whether it is viable. At best you can formulate a hypothesis about what set of features you think is good enough in terms of KPIs and then plan to build only that. However, you have to factor in the likelihood that your hypothesis is wrong and also that the process of building something should result in refining that hypothesis over time. If that doesn't happen, you are not learning and you are probably not really building a viable thing.
The fastest way to get to viability is to take baby steps: short sprints/iterations, ship often, re-assess where you are every step of the way. Do the most valuable/risky/uncertain things as early as you can so you can adjust course if your assumptions about their value turn out wrong. Most startups get this wrong and fail for this reason because by the time they figure out they are on the wrong track they've already wasted most of their seed funding on building pointless things.
The lean movement tends to focus on the M part too much which has a built in risk for products to be unexciting and ultimately non viable. It's great if you are copying somebody else's business model or building some kind of market place. It's not so great if you are trying to do something new.
Lean has a tendency to postpone value creation until you've built a lot of low value commonalities like a login system, user management or crap like that that every startup seems to spend ages on without getting it really right. If you hear the words MVP and Android IOS and web in one sentence that translates as we're building a lot of common functionality three times and all our experiments have 3x the cost. The chances of that being utterly unremarkable and non viable are huge.
The minimal thing would be to postpone that stuff until you have something worth logging into and worth having multiple implementations of on multiple platforms. Building a good mobile experience is a huge investment. Don't even think about it until you have something viable.
You are not proving the viability of a login system with your MVP nor are you proving the viability of a slick IOS experience. Your thing is viable if despite obvious UX and feature issues you still get positive KPIs. Once you get there, you can justify the expense of making it better. So spend as little time as you can on stuff like that instead of making it the top priority in your first iterations.
As long as you are doing minimal things you are not actually creating a lot of value. You are actually postponing value creation and viability. It's the hardest things that create the most value and that are the hardest to plan and the easiest to postpone when planning. Therefore I believe, Scrum is the wrong process for building something that has a high risk/reward balance and the right process for building something that has low risk/rewards. All the value is in the stories that everybody struggles to estimate. Scrum results in most resources getting sucked up by the least valuable stuff.