Replace/Clad Newel Posts

I want to replace my current 200mm wide stair rails with a top rail and glass panels, however, the newel posts need some work. The bottom one has a noticeable twist from about a third of the way up and all the newel posts are in a pretty knocked about condition. In addition, the current rails will have to be cut off and I may struggle to hide these areas on the newel posts completely. All the stair woodwork is going to be white gloss. It seems my best options are:

Cut the current posts down and attach a new upper section by dowelling and glueing. My issues here are that I would preferably like the keep the current square style posts and I would struggle to get an upper post that exactly matched the stump. Therefore it would be better to have some sort of turned portion at the join. How do you ensure that the drilling for the dowel is exactly vertical, how do you ensure that the cut is exactly square and how can you dress the top of the stump so that it fits in with the new upper section? Another issue with this solution is that I will have to slice a new upper section lengthways to replace the half section on the landing that supports the other end of the landing rails.

The other option which was my favoured path was to clad the three posts. As the woodwork is going to be painted, there is no need to use decorative wood so I could presumably use ply or MDF. However, from what I have read, it is recommended that the cladding is joined by mitre lock joints at the corners and although I have a router, I do not have a router table which, I believe, is essential.

Any advice on the best way ahead would be welcome.

Have you really considered all the options of working with the original newels. Can you not remove the 200mm rails and splice in new timber to fill the mortice holes? I often remove old mortice locks from doors and splice in a timber insert. You need to run the grain of the new insert the same way as the timber you are repairing. If you leave the splice insert just a fraction proud of the face of the newel, when the glue is set you can plane level and sand it smooth. With some fine surface filler and painting you will never know there was a repair.