To correct my false interpretation here: I recently stumbled upon the Book of Necrus and the Scales of Chassarth, so these have been named after contest winners and implemented in patches,right?

I looked through alot fo that forum you linked, its somewhat readable through a translator. Pretty amazing stuff, even after 100s of hours, I still feel like a total newbie compared to most of those guys.

Is there ever a time when using a hero to explore, rather than do something else like attack sites, is actually worth it?
To me this feels like one area of the game that's a little off balance. I mean, even when I discover that there is a reasonably rare resource on a tile, the time taken to explore and find it can be so long that it still feels like it's not worth it. In that time I could have attacked many sites and made whatever money I need to just pay for that resource when it's required. Maybe in the very late game having the more rare resources is worth it, but the system seems to undermine resources a bit perhaps?

There certainly are. You will run out of sites if you don't explore occasionally, and the reward for highlevel sites are well worth exploring for. Just don't overdo it...

As Kazper mentioned, you'll run out of already discovered sites to clear, at which point the only way to find new ones is to explore. As to resources, how beneficial exploring to find them is depends on how much you use the resource, and it's actually worth it more for the "common" resources, such as wood or iron (if you don't have them). This is because lacking those common resources can make hiring even basic troops extremely expensive as the game progresses (e.g. 300+ gold for a swordsman, vs 60 if you don't have to buy iron). For the most part, though, exploring is a way for your heroes to find sites (and thus collected gold, gear, and experience) once you've cleared all the previously discovered sites you can handle and don't have any more provinces to conquer (and don't yet want to get into an all-out war).

Actually Swordsmen only cost 45 gold if you have iron. Problem is that the game starts at like 10 gold per Iron almost right off I think (forgot to check). Considering that you need to buy the Forge (1), and Swordsmen School (2) and then buy 3 swordies right off that is a total of 5 Iron you are paying for. Then keep in mind that depending on the amount of other players that price is going up each time someone uses iron. On a 3 player game by the time I bought my 3 guys on turn 2 Iron is now upto 22 gold each so that swordies are now 67 gold each.

Early on I say yes take the time to find Wood, Iron, Mandrake and Horses ASAP. This will let you easily build up income buildings, troops, guards and even equipment cheaper in your stores. Remember that Iron, Wood and Mandrake will appear by 40% in their respective provinces if they appear at all so you really only have to spend around 3-6 turns usually and you can do this in between fights so that your guys heal up a bit.

Iron Mine gives 5+2 Gold per turn. Even without minerals it gives Hills +5 gold.
Sawmill Gives Forests +5 Gold and +2 More with Wood.
Also Stables increases Horse Income by +5 as well as letting you travel a lot faster.

All these become quite expensive without the resources though.

There is no hard rule because every tactical map is random but just remember that you will probably want to raise a second hero at some point & he will benefit from having one or two simple quests/ areas... presumably he will have better access to gear.

This is exactly why my first hero is Wizard because they benefit the LEAST from equipments. Every other Hero, especially Warrior and Scout, get a super amazing boost from gears that they are much easier to level up ounce you have them.

I just recently in a single skirmish, avg size 3 players on expert, gotten my first ever Cloud of Terror scroll. God that thing is insanely good. It's actually kind of unfair lol.