Yes, I think having an instance parameter for circuit numbering would solve that problem. I'd have to try it out and think on it some more. But I think that would work.
Yes, you can have phase-selectable panelboards in real-life, and they are used often. Honestly, I don't know what Revit thinks is the difference between a switchboard and a panelboard other than a panelboard has the ability to have schedules with two columns. I don't see why there need to be different designations/requirements within Revit, unless there's something I'm missing. So, really what I'm saying by saying you can have phase-selectable panelboards is that you can both have two columns and have the need to phase-select.
A two-column phase-selectable panelboard is typically scheduled such that each circuit (regardless of number of poles) occupies only one row (i.e. Check the "Show circuit number on one row for multi-phase circuits" box in the schedule template), and then the loads are placed on the phase we want. That's how we'd do it in Excel, at least. Typically, then, we wouldn't want a three-pole circuit to be numbered 1,3,5, but rather just 1. Then some people number down and some people number across.
The lack of a two-column phase-selectable panelboard hasn't been a huge issue in Revit because in practice we usually use them very similar to regular panelboards, especially for new construction, which is usually all we use Revit for since Revit can't handle phasing of electrical circuits.
(Edit: regarding differences between panelboards and switchboards: there are differences in how panelboards and switchboards should be calculated, but Revit can't handle all the nuances of switchboards. Switchboards can have different ratings for horizontal and vertical busses, each section can be configured differently, etc. Revit can't handle all the nuances of switchboards, so if we have anything particularly complicated, we have a 'dummy switchboard' that contains just the geometry for modeling purposes, then we have 'phantom equipment' that we host to the switchboard to which we actually circuit and schedule from. This equipment is basically just an invisible void form. I wouldn't expect Revit to be able to handle all the nuances and permutations of switchboards - the options are just too numerous. But we really do need to be able to select what phases circuits are connected to.)