I can't answer number one with any authority, only guessing, so I'll skip that one.

Number two has many of free answers, including Fusion 360, which is free for hobbyists and even commercial users below a rather generous limit value. For your application, Fusion 360 would be ideal, but has a bit of a learning curve. Many tutorials are available to make that task easier.

If you have a 3D printer, you may have recommendations from the manufacturer. Free slicers are available, Cura, Slic3r, Matter Control, others. I use Simplify3D but that's a purchased program.