I’ve recabled 100s of bikes with internal cable routing including Planet X with internal cable routing issues.
as long as you can mount a hard point at each entry/exit point, and fill the space with a low friction internal routing sheath, you should be able to get the front and rear derailleur systems to work reliably.
This may require some frame modification, but as they claim the frame is out of warranty, nothing to lose?
However, if there is internal debris or obstruction caused by a poor quality layup (i.e. random fibres, layup bladder, paper, etc.) you may find your effort thwarted as this can cause friction and prevent a clean routing path
a really good starting point is a bright torch and dental mirror or in the extreme case a probe for a video source so you can have a good look inside the frame
its surprising the mods I have had to do even on brand new “big brand” frames to get cabling to work properly; this is widening for a supposed “Di2 compatible” frame just to fit the darn connector through the chainstay