Minecraft – Is the Nether infinite

Whether you use lava buckets or redstone to obsidian, my recommendation for the actual mining part would be to get a copy of InfinitiesLoop's controlpack mod.

It contains a couple features which will help, specifically auto-run and auto-mine. With one keypress each you will be able to mine and move without any further input. It's a client-side mod (it'll work fine with your server) and works with mcpatcher.

Also, making a line of obsidian to mine is FAR easier and less time consuming than making a column.

One last suggestion is to consider building molds for your lava blocks wherever you want obsidian and then cooling it in place instead of making it somewhere else, mining it, and then placing it. Forming it in place with molds saves me a lot of time when I'm building obsidian structures.

Minecraft subscribes to the "Flat Earth" theory. Discounting mods, you can never circumnavigate the planet, as the "planet" will simply expand longer and longer in whichever direction you travel.