In order to "visit" a World Heritage Site, does an individual have to "see" the site, or can the individual just simply visit the location of where the World Heritage Site is? For example, under the basis that each seperate linear location has to be visited in order to "visit" a World Heritage Site, can an individual simply show up to each linear location of the Prehistoric Pile Dwellings of the Alps without actually seeing the reminants of the dwellings, or is there something in addition to visiting each "invisible" dwelling to count a visit?

There are no "rules" per se, but I think most of us will try to visit a visible part of the site. For the Limes that's no problem, Hadrian's Wall has several visible parts. In Germany you could visit Saalburg.
For the Pile Dwellings, northern Italy seems to be the best place (Fiave)

I think we all have different criteria for when we count a site as being visited.

For me it is all about intent and whether I feel like I have some sort of understanding of seeing something.

For example I count the Struve Geodetic Arc as being a site I have 'visited' but all I did was visit one point on its 3,000km course. (should update my review as I say there I wouldn't count it)

In comparison I have walked through Carlton Gardens and seen the Royal Exhibition Hall in Melbourne, but I only know this from looking back at maps and plotting the record shops I was visiting so I paid no attention to the Outstanding Universal Value of the site. Thus I don't count the Royal Exhibition as a 'visit'.
A bit frustrating as it is a long way to go to see what seem to be a rather poor WHS, but at least the coffee will be good! I guess if you can walk through a site without noticing it, it may not be of real OUV to you.

I'm sure others have lots of stories of their own near misses, but it basically come down to your own feeling on it. I have never felt it to be a competitive thing of accruing site visits so the only person you have to satisfy is yourself really.

In regards to the Pile Dwellings, I think I would count it if I saw just the ones at the Molina di Ledro museum. But then if I spent an afternoon traipsing through fields in Germany or Slovenia and got a good feel something there I could happily count it then as well.