How do I import an advanced auto-generated rig to Unity? My question is fourfold.
It is about blender's own Metarig, Lucas Veber's (Artell's) Auto Rig Pro and Blender's Blenrig.
1) What is the difference between Metarig, Auto Rig Pro and Blenrig?

2) I intend to use one of the following to rig video game characters (both humanoid and non-humanoid), which would be best suited for the task considering that I would like to add an extra bone to the rig for parenting weapons and tools to?

3) I have mocap data for humanoid characters, which of the rigs can best pick up that data?

4) (a) As far as I know all the above rigs generate extra bones for advanced body control, will that be too heavy for a game engine to handle considering that there may be up to 2000 characters on screen at a given time? (b) And if so then which extra generated bones can i delete from them without messing up the animation? (c) Also settings should set to export them as FBX?
1) Metarig (Rigify, actually) is a good efficient rig, AutoRig Pro is fast and fancy, but it's not very "solid", it has some limitations and some techincal troubles that I personally find quite annoying, and Blenrig is more cartoony squash and stretch oriented.

2) Rigify is the simplest, but you can perform edits on every rig, as soon as you understand their structure

3) Only Auto Rig Pro (out of thoose 3) has a built in retargeting panel, but there are many other ways to retarget an animation on a new rig (addons, manually, ...)

4) the usual workflow is: perform the animation, select in pose mode only deform bones (in Rigify they are all on the 30th Bone Layer), go to Pose_Animation_Bake Action to bake their action into a new action (Only selected, visual keying, remove constraints), and export that action along with a rig composed by deforming bones only (as Fbx works quite well).

Note: Blenrig uses lots of Blender tricks (Lattices, Shape keys, ...) that make it hard to export, if you don't want any headache try to use animations bone made only.

So short answer is: Rigify is the best for your tasks, unless you want to use the Auto Rig Pro retargeting features.

Use T pose as rest pose for best results.