This is quite a complex scenario. Your stage in customer development isn't clear to me - I wonder, how can you have an MVP if you've done no customer development?
If you still have some major assumptions to test about the customer segments you're targeting and the problem you're solving for them, your focus is "customer discovery." That is, your immediate goal should be to get that feedback. Trying to build any MVP at this stage is premature and puts all your effort at significant risk of not having market traction.
If you've reached the point where you and potential customers have isolated the problem and they have described ways they've tried, and failed, to solve this problem themselves, then you can start to look at validating your MVP.
I would test that small feature set as if it was your MVP. After all, your MVP will change as you learn. This will also give you base from which to test other MVPs.
Whether or not to charge is a separate issue, and your time will be better spent dealing with that when you are validating your MVP.
Hope this helps.