1. I have a feeling that in the past gold piles were giving more money on average. I got this feeling from watching videos, they had even some +7's there if I'm not mistaken. I usually get 2-4. Is this true, have there been any changes there, which could make some advice given in the past more or less obsolete?
No, gold piles have kept the same values since the alpha. Only black market changes them.
2. Advice goes in the direction of Thief being the best choice because of 33% more items on the map, meaning 33% more gold. However does this mean that there is also one extra shop in the dungeon? Is there a race/class that is preferred for this mission?
Nope, thief affects items, powerups, and glyphs, not shops.
3. Map selection. Is there any map that is better than others when it comes to beating the Dimensional Dealings?
Multiple boss scenarios, since that's extra gold drops. Rock Garden has been suggested due to its large number of bosses, but anything with more than one boss should work fine.
4. Preps. Obviously Black Market is the choice, anything else that I had been missing?
Tikki Tooki, can't say enough good things about that guy for this quest.
5. Obviously something like Gloves of Midas would have made this mission much easier. However at this stage I find it impossible to beat the last boss in the challenge. Is this the way to get the Dimensional Dealings done or is it possible to beat it even without Gloves of Midas? And by possible I mean better than 1 in 1000 shot.
Gloves of Midas are nice but not necessary for this challenge. Given the difficulty of unlocking that particular item, most people do not
have the gloves when they beat this challenge.