The arm looks great! When you say "Fishing line" in your video I hope you mean braided line and not a nylon monofilament? Nylon is going to be far too stretchy and have too much creep to work at all I think so it must be braided, can't tell in the video.
The limit switches I use are these guys:
https://www.amazon.com/gp/product/B00LV ... UTF8&psc=1This is a common limit switch size so you can probably find them elsewhere also. They are actually the same as the limit switches used in most mouses! I repaired one of my logitechs where the click was going out with a spare one haha.
I definitely wrestled with the "walking off the pulley" issue. There are two things that help here:
1) make sure you have a string with diameter 1mm or less (the reason tightening helps is it reduces string diameter) I tested about 7 different types of braided line and found this one worked best for my machine:
https://www.amazon.com/gp/product/B011D ... UTF8&psc=12) make sure that you have a good balance of distance between each of end of the string coming off the pulley. I had to adjust the slack on the fixed end vs. the end I reeled in with the tuner to make sure I didn't walk too high up or too low on the pulley.
One design element that also helps not to walk off the pulley is increasing the distance between the pulley that feeds the spool and the spool itself. This requires arm redesign and reprinting though and I got too lazy to figure it out. I get the full range of motion out of the arm on my properly adjusted arms though but it would be nice if you had more room for error.
The mystery hole you mention is a screw hole for my anchor of the end of the string not attached to the tuner if that makes sense. There are other places you could affix that end of the string to but I liked having that one there for neatness.
Printed spools should probably work but may introduce some error. You can get a set of 3 well machined aluminum pulleys at the bottom of this page for $20 if you find the printed ones don't cut it:
http://www.tridprinting.com/Mechanical-Parts/Excited for your build keep up the good work! What board/firmware do you plan to use?