Yes you can use the wet/dry as a sump on the 75. Remvoe the bioballs and the wet/dry becomes simply a wet (ie sump).
The top looks like an UGF
but the materal looks like it not a plate but some kind of filter mesh. Im guessing it fits the bottom of the tank and therefor is useless to you.
The parts image. The two foam blocks on the right are for the wetdry. They would fit in the bottom where there are baffles. The sponge could also fit in the tanks overflow.
The white plumbing with the black adjustable nozzle is your return line from the sump. The adjustable nozzle is to adjust how you want the water to come out.
The skimmer is at the top of the image and the J tube lookign thing is for the skimmer. Its hard to tell exactly what the black cylinder objects but they go with the skimmer.
The rest looks like misc plumbing parts.