1. Reduce your screws to 8mm. 10mm will bottom out in the channels.
2. Cast corner connectors should be fine. Strength is more than adequate and they have a dull matt finish that shouldn't cause objectionable reflections. If you do think there will be an issue, Krylon will easily solve the problem. Note, you may need to file the tabs off the bottom of the connectors to seat properly against the face of the extrusion.
3. Can't offer a specific answer on the motor question but be sure and get one that is heavily geared to reduce speed. Every turn moves the gantry 8mm so a 1200 rpm motor will get you in a world of hurt very quickly. The other problem with motors is they spin freely. The gravity load of the camera on the gantry may cause the lead screw to turn on its own so you'll want a motor with sufficient gearing that it won't move easily. Stepper motors are designed to stay put but the aggressive lead screws that come with the actuator assemblies have been known to overpower the stepper motor's detent torque. Note, you'll need to find a way to mount the motor as the actuator kits are specifically designed for NEMA 23 steppers.

 

I haven't bought one in a couple years so I can't say off the top of my head. But according to the parts list yes, although you might want to confirm that with the shop Contact Us : SUPPORT (openbuilds.com).

BTW, the shop is closed on weekends so it'll be tomorrow before they get back to you.

 

Those were our previous generation switches, still documented here docs:blackbox-x32:connect-micro-limit [OpenBuilds Documentation], but not recommended for new designs

Normally one does not need diodes - so no idea what the use case in the picture was, and you would not need it if you wire your Xtension Limits as Documented in the BlackBox Documentation: docs:blackbox-x32:connect-xtension-limit [OpenBuilds Documentation]

 

my guess is the switches were switching power to the DC motor, the diode provides automatic 'can approach and stop and can then reverse away but not drive the wrong way'