1- What is the default reference plane, i.e., the one (or ones?) used when no reference plane is determined in catalogue files? I already know that for stuff orbiting planets and moons it's the equator of the body being orbited, but how are things set up for stuff orbiting stars and other stellar objects?

Default is equator. For moon this is a parent planet's equator plane, but for stars and planets this is the Earth's equator plane.

To make a planet orbiting in the equator plane of the star, you must make star's rotation axis tilted by planet's orbit inclination angle (Star Obliquity = Planet Orbit Inclination and Star EqAscNode = Planet Orbit AscenginNode).

Wait. Earth's equatorial plane, or Earth's orbital plane, a.k.a. the ecliptic? Those are different by 23º.

... which can be quite a pain if you're dealing with stuff set up to any kind of Earth-based reference. If you want a planet with an orbit inclined, say, 10º relatively to the local orbiting plane, the math tends to get pretty cumbersome pretty fast. It'd be a lot easier and straightforward just to reference stuff to the local star's equatorial plane, IMHO.

Of course equator. Astronomy uses equatorial system.

Astronomy uses equatorial system.

What exactly does the "lifetime" section in a planets wiki window mean? And why does it always say infinity?

Try just move ship_warp.glsl to another folder

Try just move ship_warp.glsl to another folder

Or simply rename it to Doc's ship_warpDIS.glsl maybe?