This is not so much a bug as some confusion from what is written in the mod description. It says Proving grounds project for an advanced set of parts once you reach plated armour. A lot of room to specialize.
Proving grounds project for the most powerful set of parts once you reach plated armour. Enough options to last a lifetime.
I think the most powerful set of parts become available with powered armour. Is that correct?

Yup, typo'd

Sparks not playble in challenge mod (no skills, no armors, no new items), how fix that?

By "challenge mod" you are referring to Skirmish mode? This simply wont work there. The custom highlander inventory slots aren't supported so there's nothing I can do about it.

None of the parts are showing up, did it bug out because I tried it with an existing save?

Most likely.

I am about to have sparks in my current campaign which started with this enabled. I will let you know how it goes.

Working perfectly. One question. If I replace a module do I lose the one I replace like what happens with PCS chips?

No, they are just utility items that grant abilities and can only be equipped in the new slots.

Very good sparks. Not too weak and not too strong.

I ran into an unexpected result. I have this mod, dependencies checked, and got a standard Spark unit. I am not using Mechatronic Warfare, but I am using the Mechatronic Warfare Perk Pack because it is required for Lord Abizi's Gremlin & Bit Modules for Modular Sparks.

I am also using Covert Infiltration with the mod, Strategy Overhaul Chain: Spark Heist.

as far as I know all those should work together. where did this unexpected result occur? by "standard spark" do you mean one with a skill tree?

The Spark is the same as without any mods.

so no new inventory slots and has a skill tree as usual? no idea what could be causing that.