Idea for a new X axis/Extruder combo

The main Idea here is to move the Extruder motor of the X carriage, but with out using a bowden cable.
I created a mockup on thinkercad to help explain the idea: https://tinkercad.com/things/9NbRtlqDhMd-reprap-x-axisextruder-idea please note that while the design appears to be horizontal it is really desinged to be vertical.
Picture:
Red: X carriage
Orange: X rails (these are supposed to be stacked vertically)
Yellow: Drive shaft
Green: gears
Purple: hot end
Blue: extruder body
Pink: hobbled bot + idler

Disclaimer: I do not own a printer, I have never worked on a printer, I have never even seen a printer in real life. So I'm asking for feedback and advice.

The idea then is this:
Move the extruder motor off the X carriage and on to the X axis ends. Connect it to a notched or keyed drive shaft that runs parallel to the X axis rails. On that drive shaft there a gears that rotates with the shaft but is free to slide along it. That gear fits in a slot in the X carriage so that it moves with the X carriage. This gear replaces the small gear in a geared Wades or similar extruder, the rest of the extruder remains pretty much the same other than being positioned so the large gear meshes with the small gear and mounting positions changing to fit a vertical X axis.

Obviously this requires redesigning most (all) of the X axis, some work has already been done on Vertical X Axis (info here: [reprap.org] and here: [reprap.org]), however all of these designs will need adaption to add the third drive shaft.

Pros:
Lighter X carriage and extruder assembly.

Cons:
requires vertical X axis (see further thoughts about this)
Heavier X axis due to added weight of drive shaft (again see further thoughts).
possible backlash introduced by the distance between Motor and filament.
only works with a single extruder.
notch shafts seam to be expensive (the 8mm one if found on McMaster was $30 each: [www.mcmaster.com])

Further thoughts:
No reason for the drive shaft to be the same size as the X shafts, the smaller it is the lighter it will be.
it might be possible to mount the drive shaft off center (next to one of the rails) and leave the X axis horizontal.
it is also possible that the drive shaft could replace one of the X axis rails already there, doing away with needing a third shaft altogether (need to find out more about bearings to figure out if that will work or not)

Questions/ things to test:
How much backlash is added by the distance between the motor and the extruder? And will this cause problems or degradation of print quality?
How do the bearings on the drive shaft hold up under the combined stress of the of the shaft rotating and sliding back and forth?
Will a notched shaft wear out the bearing because of the edge were the notch is?
Is there some other shaft profile that will work?
On printed gears can the part that fits in the notch on the shaft be made strong enough to prevent breaking?

After doing some more research yesterday I stumbles across some earlier work that is very similar: [garyhodgson.com] which came out of this idea:
[www.thingiverse.com]

Differences are:
square rod used for drive shaft,
rods mounted in an L configuration,
it exists in the real world ,

thoughts that should be considered:
they were thinking about possibly of doing away with the bracket for the driving gear, I was thinking of using flanged bearings on both sides of the gear (something like these)
their design (as far as I can tell) was never tested extruding so we have no idea as to whether or not the gear will slide when stress is applied.