Concrete – Rust free concrete fasteners

(I'm assuming that you are in the UK).

Concrete floors without DPM are not unusual in houses of that age, so there is a possibility that it's not a cowboy job. The broken bitumen you mention was probably the remnants of an asphalt DPM over the slab. The wood has (as you suggest) likely been used to level the concrete. If they were previously joists for a suspended floor, some have certainly been removed, as the spacing is way to wide for floor joists.

Without ripping the floor up, I can see two basic options:

1: Fill the gaps with concrete and get an asphalt company to lay an asphalt screed over it to restore the DPM.

2: Fill the gaps with concrete, lay a DPM over it, insulation, then a concrete / fibre screed. Note though that this will raise the floor level by 75-100mm (depending on the insulation).

In terms of concrete, as it looks like it's not a structural element, you could just use a "basic" 4:2:1 concrete mix (4 parts stone, 2 parts concreting sand, 1 part OPC). Add enough water to make it workable, but not so that it gets sloppy.

In general, paints like this do indeed stick to the rust just fine and encapsulate it by linking together and forming a hard surface. This is good if you want something to look good, but it doesn't do much to stop the rusting process. Eventually, the metal below the paint will rust further, and in the process expand. This will cause a crack in the paint, and eventually it will chip off. If the steel angle in question isn't structurally important, that might be fine. Depending on your climate, painting over it might buy you a few months or a few years. If failure of the steel would be a big problem, then you should sandblast or otherwise get to white metal, and pick a coating system designed for rust prevention.