I just hope all the Deep Ellum opposition realize that all of these 'subway' options still have it at grade well before it reaches DE.
IMO, B7 makes sense as a subway. If you're going to shift south to C3A, you may as well go a block further to Canton and keep it at grade all the way. You could leverage the existing track all the way through the CC station, and save a ton of $. There really isn't a lot of traffic to disrupt, or neighborhood to split down there, but a couple stops may bring some new low/midrise apartment development. That alternative would cost pocket change (relatively speaking) compared to subway, and it could be build with almost nobody being disrupted, and in an incredibly short time frame.
We have to keep in mind that Subway construction, particularly at shallow depths as would be the case here, cause serious disruption to the neighborhoods above for several years. When LA built their subway, it killed the street life and business, and took many years to recover.