coral in freshwater tank....

No, you won't really be able to control the pH. You can allow the tank to stabilize at a higher pH, and possibly keep it a shade lower by having lots of wood in the setup, but it's going to make it hard to have the change water similar to what's in the tank. If you want to go with fish that prefer hard water/high pH, it should be okay, too.

Personally, I wouldn't add it. For one, those items may have been removed from the living reef to be made decorative, bad for the reef. They may have been collected loose, too-=but it's hard to know. Second, I don't like the FW tanks that have SW items in them--just looks weird. I'd rather just have the SW tank.

Many African Cichlids prefer high pH, hard water. Lake cichlids, mostly.

Hard to guess--a lot will depend on where your parameters start out. You can test--fill a bowl with tap water, let it sit out overnight, test. Add a chunk, let it sit a week, test.

Depends on what animals you want. Simplest, tank, pumps, light, canister filter, and marine mix of course. More complex--high output lighting, live rock, sand, marine mix, filtered water source, etc.

Marine mix is the salt mixture use to make saltwater for the tank. Marine water is more than just salt, and the prepared mixtures add the trace minerals and buffers needed to closely replicate actual saltwater.

I would not add coral to your tank. Any polyps that are left will decay and poision tank. VERY hard to remove all dead matter from dead coral.

Those look like the bleached coral skeletons sold as tank furniture for pre-reef era saltwater tanks. They should not contain any decaying polyps. I would guess thay would adjust your ph to around 8.0 (just a rough estimate mind you)