There are three reasons to use a rear-mount parting tool. A threaded spindle (risk of unscrewing), no spindle reverse, and the risk (perhaps exaggerated, as I've never seen an actual report) that a front-mount puts so much stress on the saddle that the dovetail cracks (cutting forces are up, not down, so aim at the weak point of the dovetail).
I've used an upside down, front mounted parting tool for decades with a D-4 spindle mount and reverse. Doesn't bind, fits my QCTP like any other tool, and everything is in clear view. No problems with this approach on several lathes. In fact, the best feature is that it fails-safe by lifting slightly in the QCTP if it does bind (leave the QCTP locking lever tight, but not too tight!)
i'm curious to know if there was a specific reason for your design choice. From the pictures, your lathe looks as if it has none of these issues.
Nice job. And a good looking lathe, too.