Dave, yes tube should be but it will depend on material of tube and material of shaft I think, certainly for same material the tube should be better.
Funny thing is boats always have solid shafts, usually Duplex nowadays.
Wont really have to worry about RunAway, AC Servos are pretty good at monitoring things and always have error signals tied into my E-Stop chain.
If I do go with dual motors I will likely just use one side as a follower to the other.
Yes it is a shame but the machine was bought as scrap to rob the control off, the trucks on the profile rails were in a bad way, only managed to get 5 rebuilt out of 12 that were on it but managed to pick up 3 new on eBay for a decent price. The rails are not the best either, one is excellent, one not too bad but the others have a good bit of pitting. Still accurate enough for a plasma but will be a lot noisier than they should be.
The rack and pinions are all good from what I have seen so far (covered in greases) so that was a bonus.
pic below of rack and pinion.
Hood