Walls – this orange yellow substance on the wooden studs inside walls

In general, load bearing walls are placed above and parallel to (with as little offset as possible) the beams below them.

I would span another beam or mini beam parallel to the first one, to support the joists mid span. I would start out with screw jacks (raising a new beam) and some form of force plate over your slab (steel over plywood to support and distribute the bottom of the jacks), and S l o w l y elevate. 2 inches is a lot of movement. This elevation process should be done over months. You may find that stabilization is all that is necessary, (or prudent). If you add 1/2 to 3/4 at your lowest point, you might be happy.

Monitor what is happening at the original beam. You may find a gap developing there.. If that is the case, you may have to raise the original beam slightly or shim the beam to joist connection with tapered shims.

Once you're happy with the final leveling, permanent columns (and footings below them) are the proper conclusion.

I suspect that the material is a lot closer to glass than resin, and is part of the "look" of this particular style of brick - that is, it came out of the brick kiln that way.

It might or might not be manganese. One of the potters I know is prone to using a light clay with manganese, which does get brown specks, but they are smaller - however, that's "pottery clay" not "brick clay" and perhaps they are smaller due to using smaller bits of manganese. Otherwise very similar in "look."

Closest I've found in a quick look is "light ironspot" - still looking for a decent picture. Name would suggest iron ore rather than manganese for a material.

In short, nothing to worry about.