Re (a): Yes you can do it that way and get the same quality with each disc. But Canuck's proposed method is better ... and faster, though you will need a separate program like Nero, Roxio or Ashampoo Burning Studio to burn the iso or folder.
Re (b): A lot of us, including me, do it that way, for the reason explained by Canuck. In a sense, though, the workflow was originally devised in part to solve a problem quite a few users had in earlier versions of VS. Their computers were not powerful enough to do both the rendering and actual burning in one process in the burning module. Separating out the two processes put less strain on their computers. But these days, most computers are up to the mark and newer versions of VS are more capable, so your existing workflow is fine. But at least you have "our" workflow to fall back on if something goes wrong. And remember Canuck's comment that you won't always be wanting to burn an actual disc. These days, for instance, I very rarely burn a disc. I now work almost exclusively in HD, and simply put my completed, rendered videos on a stick drive or external hard drive, connect that to my PS3 and play them directly to my HDTV.
Re (c): Sure it makes sense. External drives are cheap these days, and X5 can handle HD video stored on such a drive. I just completed a project yesterday doing just that.
Re (d): 1920 x 1080 is currently the top resolution in the consumer world. And I would certainly recommend rendering your AVCHD projects into a new AVCHD file. AVCHD is one of the compatible Blu-Ray formats, so when you eventually buy a Blu-Ray burner, you can burn them to disc. In fact you can already do so by burning your new files to a DVD but in Blu-Ray format. You select Share > Create Disc > AVCHD. An AVCHD, or hybrid, disc is a Blu-Ray folder burning to a standard DVD. To maintain high quality, you can only burn about 20 minutes of AVCHD to a single layer DVD, but the quality is excellent. What's more, you don't need a Blu-Ray burner to make such a disc, but they can only be played in a Blu-Ray player which accepts hybrid disc - though most do these days.
X5 can also handle full progressive 50p video, though you need to enable this under Settings.