Why would removing the fork damage something? It is a relatively simple process for either threaded or threadless. I'm sure most bike shops would happily remove the fork to get at some debris for a nominal cost if you don't want to do it yourself.

With regard to internal cable routing, plenty of online guides, like this: Ask a Mechanic: Replacing Internally Routed Cables

A google search will show you others as well. Park Tools also makes a magnetic cable routing kit which can be useful.

The nylon guides might just be there to prevent road grime from getting into the frame easily, which is not really an issue on the top tube. If you like the bike I would just get it and have a bike shop try to get the debris out.