To answer your questions:
My primary use case for this is not as a data export. While that could be useful in some situations, I mainly just want the visualization if I had to choose one or the other. (though I could see the power company wanting an export to prove it to them)
I am not sure if we would want to overlay this on the usage screen necessarily since that may make it too complex, and the primary focus of Sense I think has been on utilization, so while I want a graph of the phase leg voltages, it should either be a separate graph, or perhaps something that could be toggled on or off along with the usage. (being able to look at voltage and consumption overlaid could be very insightful, but I also see the need to look at each separately)
I should call out that on the main usage graph, I really also want to see the two phase legs consumption able to be displayed as a stacked graph or something (maybe this is an “advanced” mode feature that could be toggled in settings). It would be really helpful in identifying some loads to see which phase leg they are on.
I very much dislike how buried the real time consumption and voltage data in Sense monitor is - it seems like making that more accessible in the app could be useful. I also think you should add a third reading in watts to the monitor: I want to see solar generation, total power flow out the electrical mains to/from the utility, plus values on each phase leg of the total power the house is consuming (so utility + solar). Power factor information (VA) might be interesting as well.
As to alerting, I would say the most critical would be around helping detect a loose neutral (or other sustained major voltage issue). So perhaps some upper and lower bound thresholds for each phase leg (probably could make them the same for both for simplicity?) that were defaulted to some reasonable levels. But let the user adjust perhaps since some folks may have really good or really bad power and want to modify the thresholds.
Then perhaps more advanced alerting would be around brief power quality events and extended issues. Things like short power blips (dropouts), overvoltages, actual outages, etc… It would be nice if Sense had some “ridethrough” ability in the hardware so it could stay online during brief issues (and still logging data about them). Then it would need to be able to queue that data up to send back to the “cloud” later assuming that often the Sense might lose its Internet connection briefly due to the issue. It may need to commit it to non volatile memory temporarily if its “ridethrough” ability was not long enough to make it through the issue. Some folks may want to be able to get a notification if their Sense unit becomes unreachable as this often will indicate a power outage.