Internal cabling is handled a couple ways- The best way is to insert a full-length steel tube, inside the TT, to encase the cable housing. This makes it much easier to pull in new housing. The other way is to just insert short pieces of tubing in the drilled holes. This method makes it very difficult to re-cable. Both would be brazed into the top tube.

My guess is that your cable isn't actually siezed in the "cable guide" from corrosion, but could be if the bike is really subjected to moisture. I'd think though that the cover on the housing is swelled, or has caught on the edge of the guide tube(s).

You don't have enough access through the headtube or the seattube to do you much good. Just grab the cable with some vise grips and pull like crazy. It'll either come out, or unravel and come out that way. Or, not come out at all, in which case you just route the cable externally.