[ortek_service]

Well the input-supply to the 5V regulator shouldn't really need much over-voltage protection as it inherently regulates to the desired voltage. And it may well withstand 30V into it - Although power dissipation in TR5 could be an issue!
However, no doubt the 30V supply wouldn't source enough current, and would collapse under the main 5V rail's current, as 30V current only needs to be enough for Vpp requirement and will be limited to < 100mA by the regulator IC in the 25V/5V Vpp reg circuit..

A small 18V zener isn't really going to provide much in the way of over voltage (or reverse-polarity) protection, although it might take out some transients (but doesn't really seem the ideal part to choose for this).

If they didn't need low-dropout, I would have expected them to just use the classic NPN series-pass circuit arrangement (or a 7805). But I think a low dropout regulator was necessary to minimise the power dissipation in TR5 as there's only a rather-small heatsink on it (And on a previous unit's photo, there wasn't one at all, unless someone had removed it).

It will be interesting to see if there''s any labels on the PSU with its output voltages / current etc. Although it looked a bit basic.
Or maybe more info in the manual etc. So may not need to actually measure it.