I recently started work on mapping my campaign world as well. I got a lot of help and inspiration for creating a believable map from this thread here. The creation process for the map in that thread covers everything from ocean currents to humidity to tectonic forces and overall it's pretty simple to follow. That should give you a good starting point for your geography and climate.
How to present your map without overloading your players? Well, I would prefer my players asking "what's over that mountain?" and not "what's on that other continent?" In which case I would suggest keeping your overall world map to yourself for reference and simply create smaller regional maps that fulfill your player's immediate needs. Such maps could be easily printed on standard paper in whatever printer you have available. This is the route I'm going. I've created my world map at a large enough resolution to be printed as a poster sometime in the future (24"x36" @ 300dpi). At that size I've wound up with an image 10,800 pixels wide, which would equate to just over 2 miles per pixel around the equator in your case. If you're curious, a completely blank Photoshop document with those dimensions is 222.5Mb. I recently completed work on the overall world map which gives me basic info on climate and terrain, but isn't at all useful for distances considering the curvature of the planet. I will probably create continental maps (still not useful for distance save for small continents) which will give me more terrain details and probably political maps. Then I'll move on to regional maps that will actually be useful for distances and will likely be more useful to my players as well.
1. For your world map I don't think you need anything larger than about 2 pixels per mile. Unless the map is actually printed on a globe, scale is actually pretty meaningless here. Just go with a nice rectangle (again, 300dpi 2:3 poster size works well and can be made into some nice wall art later). I would consider only marking truly major cities, roads and landmarks on this scale. Again, this will mostly be for reference as the scale of the map won't be very useful to you.
2. Having never used such techniques myself I can't really advise you either way on this.
3. Yes, make multiple maps. A world map is good for your own reference, as are more detailed continental maps. Continental maps could come in useful for players and definitely make regional maps for your players to use.
4. Could help for making more accurate maps at smaller scales. In Photoshop I took my world map and created a 3D sphere layer using an image of the map. It really showed me what mistakes I'd made in the map creation process and allowed me to see exactly how everything came together at the poles. Very very useful for a map-creation reference. I don't know if GIMP has an equivalent feature, but I would recommend putting your map on a sphere early in the creation process to ensure everything lines up and that you understand how your world fits together.
5. I don't think you're far up a creek at all considering you're still in the planning phases. Even if you are far up there you most certainly have a paddle or two.
Hopefully all that helps. I fear I repeated myself a few times, but those are my thoughts for you.