First: the thread should be moved to Products because it is Marketo-product-specific (as opposed to a question across the marketing discipline). I know this is confusing and I wish the Places had better names.
To your post: I think you're skipping past a couple of questions about Marketo's recording and reporting capability and need to back up a bit. You seem to be talking about a form that a person may never completely post, that is, the form will be abandoned and thus a corresponding Marketo lead will never be created/located. This means you'll want to report on anonymous leads, which is not a good fit for Marketo: the platform has robust support for displaying/reporting on earlier anonymous web activities once a lead is known, but when a lead is still anonymous reporting is sorely limited (it used to be less so, but now the direction of the platform is clear).
On the other hand, if you redesign your concept so that the lead enters their email address first, and an initial form submits in the background with just their email address (while they continue to fill out additional fields), then you would be able to see all their other actions -- whether they formally "convert," using your rules, would mean they purposely click Submit later on.
If you want to slice-and-dice by mobile vs. desktop, it isn't necessary to have a separate LP, though it would give you an easier visual breakdown (like on the LP statistics page, where you'd know that anybody who hit a certain page was mobile).
I feel overall that you need to decide on the reporting goals first. It's good, if you're not completely familiar with the options, to take a reports-first approach to your design. You don't want to gather data but have no way to surface it without using a separate BI tool (if at all).
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