eldrick

Right-click any STL file and choose Get Info. On the info panel note the "Open with:" item halfway down – choose Cura, then click Change All button. Done.

For several years I have heated the bed up to the target temp, while heating the extruder to about 50C less than its target. This avoids the possibility of overheating the material in the nozzle, and doesn't heat both at the same time for nearly as long. I've never had any sort of issue from doing so. I usually do this manually, rather than in starting gcode.

Adjust the Line Width by a a few hundredths of a mm up or down until you see the long gap vanish, for a start, and it will reduce the triangles size.

Windows

sketchup does a lousy job of exporting 3D files in useful formats for 3D printing. Use almost anything else.

I agree with Smithy - use a high number of walls, like 50, so it prints solid. In any case, a small printed plastic screw is unlikely to be strong enough to be useful, because the unthreaded core is only around 8mm at most.

The problem with the 1,2,3,4,4,3,2,1 sequence is that parts one and four are going to have two layers printed in sequence, and for small parts the first layer won't have time to cool before the next one starts, which often affects print quality negatively - it can leave globs on the parts at the beginnings and ends of the sequence. Several slicers have used this sequence in the past, and most have stopped doing so because PQ is more important than time - it's a bug, not a feature…

Your problem is using Sketchup, which is ill-suited for 3D printing.

For what you describe, you don't need two nozzles. Merge the base and text into a single piece, then start printing with the base color, and when the topmost base layer is complete, clip the filament just above the extruder and follow the filament in with the text-color filament so the same extruder begins printing with the second color.

I completely agree - that's the only way to avoid lots of re-prints and poor quality printing for that piece.

You might also verify that Cura is set to the correct filament size, i.e. set for 3mm filament rather than 1.75, if that's what your printer uses.

Why are you wasting time trying to use obsolete v2 of Cura, when the rest of the world is on v3.1?

"Any suggestions?" Go to any hardware store and buy an M6 machine screw instead of printing it.

Or if you need fine control, Slic3r offers manual control of layer height.

I use fuzzy skin for my product, and would appreciate not having it applied to interior surfaces. Being able to choose the surface elements to randomize would be the best solution for my parts - specifically, I'd like to exclude the center hole.