1. - editing auto-routing. On the way home from a ride yesterday I tested the auto-routing feature which appears to work like a car satnav and works out the route for you. Can I add via points to the route or do I have to persuade it by riding off the advised route to the place I intend to via through - not very useful in unknown territory?
Yes, by creating a "Route", either in on the device (deeply tedious) or in Basecamp or similar, consisting of the points you want to pass through. To follow it, you select "Where to?" -> "Route" -> the name of your route, and if it asks whether you want to navigate on or off road, select one of the on-road options. It will then automatically calculate a route that guides you through the waypoints from your current location, with turn by turn instructions.
One gotcha is that it's a bit clever, and if you appear to have skipped some routepoints, it will take you to one later in the route if that's nearer. So if you make a circular route, it's possible that it'll take you straight to the end from the start (which isn't very useful). To avoid this, I recommend making circular (or overlapping) routes in more than one Route.
The other gotcha is that it only has the data in the map to go on, so like any other sat-nav device, it won't always make as intelligent routing decisions as a human with local knowledge. For example it has no idea about Bastard Hills, and thinks cyclepaths are good for cycling on. You occasionally come up against quirks in the map (particularly with OpenStreetmap in less populated areas), where junctions don't 'join up' correctly in the routing data, and it will refuse to route you in an obvious (to a human) direction. Given sufficient hand-crafting of Routes in order to force its hand with routepoints, the auto-routing can usually be persuaded to take you on a desired route, for turn-by-turn instructions. If you do this, I recommend having a visible Track (the "show on map" option - literally just a line on the map) of your desired route at the same time, so you can see if its leading you astray.
2. Proximity alarms. I can add a proximity alarm to a waypoint created but this seems to be a laborious process if I want to do multiple alarms as I can only do them one at a time - this might be a function on Basecamp but I haven't used that enough yet for route creation (is it)?
The interface is less tedious in Basecamp: You open the waypoint for editing, and one of the text boxes in the relevant window is the proximity radius. You're still doing them one at a time (but you're probably creating waypoints one at a time anyway), but it's much much quicker than on the device.
3. I think I have read elsewhere I need TCX files to get proper turn by turn instruction on a 'route' created but I cannot load a TCX into the Etrex 30. Is this the case?
TCX is a weird Edge thing, not relevant to the eTrex. Don't worry about it (at least until someone wants to share data and sends you a TCX file, in which case you'll have to convert to GPX). For turn-by-turn instructions, create a Route in Basecamp.
