Are you using emoncms.org or a self-hosted emoncms?
The answer to Q1 is, the the apps module only officially supports phpfina feed types but it is easily altered to use other feed types. I use other feed types successfully but have never tried a virtual feed type so I cannot say if it’s possible or not, See the Does emoncms expects data to be send reguarly? thread for a discussion on how to change the supported feed types in MyElectric, a virtual feed is type 7 if you wish to experiment on your self-hosted emoncms, on emoncms.org you have no option but to use the supported feed type(s).
However, regardless of whether that does or doesn’t work and to answer Q2, the simplest way is to use input processing to arrive at a total and “log to feed” and choose phpfina feed type, this does mean using more disk space but the phpfina feed type is quite small on disk and much faster than a virtual feed as you are querying persisted data rather than calculating on the fly with each query. I keep several phpfina feeds for no other purpose than viewing in MyElectric etc as it’s quicker and I’ve plenty of space as I use hdd’s not an SD.
If you do try using virtual feeds in my electric, please report back on how it goes, as it would be nice to know.