Can't comment on the Rakiura, but...
Given the choice between the Routeburn and Kepler I would definitely choose the Routeburn. It's shorter but can be tied in with the Greenstone or Caples and to me has better views for effort. Both are good, relatively easy walks and both easy to arrange buses to/from the Milford. Day 2 of the Kepler (if doing it anti-clockwise) is spectacular but the rest of it is pretty straightforward, whereas I'd argue that the entire Routeburn is pretty jaw-dropping. Others might have different opinions though.
dinna89 wrote: I'd also be looking to do ... Routeburn in 2-3 days (1-2 nights), which seems to be achievable based on lengths and elevations.
2 nights is the norm, but could do it in one if you wanted to push it. Personally I'd rather spend the extra time to soak it all in. If time is an issue you could do get the bus from the end of the Milford then get dropped off at the Divide - maybe an hour on the bus? - and walk in to Lake Howden or Mackenzie that night and finish at the Glenorchy end. That would give you enough time to take it easy on the Routeburn and still have a couple of days in Q'town afterwards.
dinna89 wrote: I'm a bit dubious about the Milford Track, I know it's meant to be amazing, but the high level of management is a bit of a turn off, with designated huts each night, and the daily waking section pretty short.
The regulation of the Milford is a bit of a pain in the ar$e but still well worth it. I was similarly dubious and had previously avoided it for that reason, but when I finally did do it I found it lived up to the hype. On the length of days, the two middle days aren't that short (especially the third if you incorporate a sidetrip to Sutherland Falls) and the first and last days incorporate ferry trips at each end, so you don't spend too much time sitting around.
dinna89 wrote: Does anyone know what would happen if I was 'delayed' and arrived a day after my booking, would I be able to undertake the first two sections in one day, or is this a terrible idea?
Not sure about this... others might know. As far as I'm aware DoC are pretty anal about not sticking to the normal schedule, but they may not mind if, for circumstances beyond your control, you needed to catch the early ferry from Te Anau and skip the first hut (which is certainly doable physically). An email to the Te Anau DoC office would clarify.