You sound like you want two things: the world should be just like Earth for familiarity / storytelling purposes and the world should be as different as possible from Earth because I AM AMAZINGLY DIFFERENT AND PEOPLE NEED TO KNOW THAT! Sorry, that last part was from too many years of too many people demanding difference in their maps that they end up never using and it was rude as well. I apologize, please forgive me.

To hit the difference part, just sketch in some vague areas in the world as you have suggested that you did and as Redrobes suggested. The less detail that you have in the big map, the easier it is to retcon things later.

For the familiarity part, think about the stories you want to tell initially and take a look at a globe (ideally a map of Earth about 1500 - the start of the Age of Exploration, before things got too muddled globally by Europeans). Find an area (or maybe three or four) that has the sorts of terrain (mountains, forests, etc.) that you want to tell stories about. You'd be surprised what you can find on a globe, especially if you're willing the fudge the scales by a bit (much above about 10% starts to really skew climates and stuff). By picking things pretty much from a real map, these local areas are guaranteed to be completely scientifically plausible.

If you have some areas that have to be connected for story purposes, you can usually paste together areas from similar latitude in the same hemisphere as long as you watch some obvious constraints (Los Angeles and Atlanta, for example, should never go right next to each other for any number of reasons).

Unless you're planning to end up with a giant detailed map of the world that's needed for storytelling, that's it. I have never really needed more than that (THEY will tell you it's because I lack imagination). Local areas for telling stories and having adventures, and top-level things for tales of far-away lands and connecting the local things. Again, the less detail that you have at the top level, the more outrageous the rumors and player-heard stories can be without affecting anything else (for example, the players hear that "The Fiddlupp that live over those mountains are like big purple octopi and will turn you into a beast of burden" when that's totally untrue because they have only one "p" in their names).

So, to take your questions in turn: yes, focus on the things you'll be using most. Add just enough glue around it to get to the next activity and provide rumors for hooks. How much higher detail is needed has a lot to do with your players, but the less detail at a high level, the harder it is to box yourself in. To be scientifically accurate, pick real-world areas unfamiliar in time and space to your players and use those. To learn about geology, study geology!

I understand the desire to go deep into the geology of an area, but unless your group is a bunch of mining engineers, you may be hard-pressed to get them to notice that there are rocks even when giants are dropping said rocks on their heads. Maybe a good perception check might have someone notice that the rock that broke open after bouncing off the mage is sparkly inside (the rock, probably not the mage), but that's unlikely to get pursued even so. With tasty monsters around, most players tend toward looting those monsters rather than looting the overall landscape.