Hey there. I started my DE army out with the battle force set as well, it was a great buy as I use everything in it regularly.
I think you are right, the Loremaster would make a very cool looking Archon with a simple Head/Weapon swap. I would also maybe either try to file over some of the High Elfy looking things on his torso like the suns on the tassets, or the heart thing on his should. You should have plenty of cool extra bits from the battle force box to put in their place, like some of the trophies from the Sybarite/Hekatrix pieces.
The Shadow warrior neck, head, and cape could be used to make some really cool Trueborn, I'm going to toy with that idea myself.
Greenstuff I have found to be easy to use, hard to master. Practice with very tiny amounts at first so you don't waste too much. Also I have found its easier to use if you dab the tips of your fingers and any tools with a bit of Vaseline, as it makes it not stick, and doesn't leave fingerprints.
As far as cutting the models both are plastic you should have no problem with simple plastic cutters. (I've even used fingernail clippers before) the hand looks extended from the body, so I wouldn't think there would be any issue there. The head may take a bit of concentration it you want to keep the collar, but it still looks pretty isolated from the rest of the model. Just replace with the ideas you already have (the Archon models arms are separate pieces, separating hands cleanly shouldn't be too tough), and I think that would be an awesome looking Archon.