Hi Deano
I'm not Bob (obviously), but I'll give you my take, for what it's worth. Essentially, you're right:
1. With a DME Arrival, you can descend step-by-step as the DME reduces towards the NDB (or GNSS fix, which should be the same point), and thereby be at MDA for the DME procedure (well below the MSA) by the time you reach the NDB. A DME Arrival is not specific for any runway (that's why it's an Arrival and not an Approach): to get to the runway, you can either follow-up the Arrival procedure by circling (if you meet visual criteria for circling), or use the DME Arrival to get you into position to start some other instrument Approach.
2. With an NDB or VOR procedure, again you're correct: you may not descend below MSA before reaching the aid (indeed, in some cases the approach may require you to cross the aid at an altitude higher than MSA, to maintain terrain clearance on the outbound leg: random examples - the NDB-A for Brisbane/Bromelton, or the NDB RWY24 for Emerald). Two exceptions that may allow you to cross the aid at lower than MSA:
a. If the route LSALT is lower than MSA, you can stick to the LSALT (if you stick to the route!)
b. If the 25nm sector MSA is lower than the 10nm MSA (which happens quite often), you can still use the 25nm MSA within 10nm, provided you keep to the sector (AIP ENR 1.5 para 2.2.1). Obviously once you cross the aid, you're in another sector: you need to either be visual; start an approach and stick to the altitudes for that approach; or climb to the 10nm MSA and start thinking about where to next.
Note also that sometimes the NDB procedure is an Arrival (e.g. Bromelton, where there isn't actually a runway), and sometimes it's an Approach (e.g. the Emerald NDB for RWY 24).
3. MDA for an Arrival procedure is always for circling - there is no direct runway approach for an Arrival. For an Approach procedure there will be separate MDAs for the straight-in, runway-specific approach (marked 'S-I' on the approach plate), vs for circling (marked 'CIRCLING'). Obviously the Straight-IN MDA will usually be lower than the Circling MDA - though I have seen a plate where the Circling MDA is lower, which seemed very odd to me.....
