As a WDW Vet, I highly recommend that you take a quick look at HydroGuy's guide to DLR for the WDW vet. It'll help you ge a sense of the parks and resort.
http://www.disboards.com/showthread.php?t=1162599
In terms of park hopping, the two parks are only 110 yards apart. I like to describe it this way: think of DL as Epcot's Future World, DCA is World Showcase, and DTD is just outside International Gateway. It's very easy to hop between the two parks. My family usually hops between the parks on all days when we're at DLR (to be honest, we also park hop at WDW).
In terms of the MHP, if you opt to go, there is a HalloweenTime SuperThread, which includes a comparison between MHP and
MNSSHP.
http://www.disboards.com/showthread.php?t=3270919
As the PP said, if you go on Monday and Tuesday you stand a pretty good chance of getting a fair amount done. I would make an effort to get to the parks at rope drop because the crowds are often lower in the morning. I do think you can get at least a couple of the big rides in the morning, especially if you make use of the FPs. Personally, I would get a 2 day park hopper plus the MHP ticket, especially if you aren't getting back to DLR in the near future.
It is worth noting that there probably won't be any fireworks (except at MHP) or Fantasmic or Mad T Party (they are not normally presented on weeknights in October). WoC will probably be presented on the week nights.
