In an attempt to answer to your questions:
1) Some of the Dyna Techs from this period were specced with Mag 21's which limits your travel fork travel options to about 60mm. Although in practice you'd probably get away with an 80mm fork running a bit of sag. Any more and you'd kill the geometry.
2) In terms of the serial number I think yours is a 1993 frame.
A=1990, B=1991, C=1992 and therefore D=1993.
However the LX Parallax hubs first appeared in 1994 which, assuming they are original, suggest a 1994 model year bike built with a frame manufactured in 1993. This isn't at all uncommon.
3) The lugs will be aluminium alloy, possibly with some steel inserts, the main tubes will most likely be titanium and the rear triangle will as you've said be cro-mo. Check everything with a magnet to be sure. Some of the higher end frame-sets also had titanium chainstays.
4) The Ti tubes will be fine left bare as they won't corrode. Just buff them up with a 'Scotchbrite' pad if you are so inclined.
Looking forward to seeing some pictures.