Yes, old maps can still be decoded. Even if they had been decoded (but not dug) before, after 30 days ALL maps reset and must be decoded again, so you'd still be ok.
Luring one mob away is pretty much the tactic now, except that the guardian mobs rubber-band back if you lead them very far (sometimes just a screen away will do it, unless, of course, you have only a sliver of health and are running like a bat out of %^&$, in which case they chase you forever. Seriously, that seems to be part of the AI, because as soon as you damage them they bounce back.) This just means you work the spawn a little closer to the other spawn than you might like. Usually not a problem except sometimes the corpses of your first kills poof before you get everything killed and can go loot.
I use the first invis only to survive the initial pop. As soon as they have popped and my RC aggros one, I fly out of range, invis again, then work it from the edges. Since they rubber band, I end up running out and invising several times as other mobs aggro me.
I mentioned that I cannot solo lvl 7 Ilshenar maps with my t-hunter. That is only because of one thing that can spawn on them: the Renegade Changling. If you have ever killed Irk, Spite or Guile while hunting Dread Horn keys, picture one of them as a Paragon. That's what every Renegade Changling is like. Which means I am always on the lookout for lvl 7 Ilsh maps. They are the only crazy, everybody dying, running for your life, hanging on by a thread, wow-it-feels-so-good-to-finally-kill-this-thing t-maps left anymore.
Even a lvl 7 Fel map with 3 Frost Dragons (beefed up, cold damage Greater Dragons) is tame in comparison.