Learn Spanish (Latin America): Que me conste, él no usó ese pincel. - As far as I know for certain, he didn't use that paintbrush.

Both phrases set limits on what you know. While que yo sepa translates broadly to "as far as I know," que me conste is stronger. The verb constar means to be certain of something or have proof of it. So, que me conste means "as far as I know for a fact" or "to my certain knowledge."

Why is conste in the subjunctive here? Is there a trigger?

This is a set phrase (a formulaic expression) that always uses the subjunctive. The que at the beginning implies a hidden negative clause like "There is nothing that..." but at a B2 level, it is best to simply memorize que me conste as a fixed chunk that limits your statement based on your personal knowledge.

Could I use brocha instead of pincel for "paintbrush" in this sentence?

It depends on what he was painting, but they aren't interchangeable. A pincel is a fine brush used for art, like painting a canvas. A brocha is a larger, thicker brush used for house painting. Because this sentence uses pincel, it implies artistic or detailed work.

Is the pronoun él strictly necessary here, or could I drop it?

You can drop it and just say no usó ese pincel. However, native speakers often keep él in these types of sentences to add emphasis or contrast. It sounds a bit like saying, "As far as I know for a fact, he didn't use that paintbrush (though maybe someone else did)."