3. What “scan tool” is available that will work with an obd1 and tell me all of that info?
I use a Snap-On Solus Pro, purchased used from a seller on eBay. $3500+ worth of top-shelf professional-grade tool when new in 2008, for $350 delivered to my door a couple of years ago. (Somewhat more than that now--$450--$500 due to Senile Joe's economic problems, coupled to the Wuhan Plague Inflation.) Before that, I had a Snap-On MTG2500 with software cartridges covering about the same time frame as the Solus Pro--1980 1/2 to 2005. (My Solus Pro goes to 2007.) Be sure to get all the OBD1 connectors you need, GM uses at least three, plus the user manuals, cables, and various other accessories. Snap-On offered updates to the software, so the same tool might have software good to 2016 or even newer. The newer the software, the more expensive the tool is likely to be, choose wisely.
There are numerous competing products, including "new" consumer-grade tools, and scan-tool simulator software-plus-connector cable or bluetooth dongle that talk to your laptop computer. I don't trust any of that, but it's a popular work-around due to the much lower cost.
The ONLY scan tools I have actual experience with are the Snap-On units I listed.
4. All of these components have been replaced, however I will recheck everything
"Replaced" doesn't mean "usable" especially with so many craptastic or outright counterfeit parts coming from Communist China.
Just as important, replacing "parts" does not fix faulty wiring between the "part" and the computer. You can replace a hundred sensors, but if the wire harness is damaged, you're not fixing the actual problem. Which is why verifying with a scan tool is much more thorough than testing a component with a multimeter--the multimeter doesn't ALSO test the connected wire harness. The scan tool shows what the computer is "seeing" from those sensors, so the wire harness is included in the diagnostics.
5. Everything involving the EGR system is new. What would be the best way to check if it is working?
Again..."New" doesn't mean "Functional".
Lift the EGR valve pintle at idle. Engine RPM should drop, maybe even stall the engine. Let go of the pintle. Engine idle should recover. Disconnect the vacuum hose from the EGR valve, connect the hose to a vacuum gauge you can see from the driver's seat. Drive the vehicle, watch for the vacuum gauge to show vacuum at moderate speeds--NOT at idle, probably not at WFO throttle, but in between it should show vacuum. Depending on the computer programming, you may be able to see this just by opening the throttle somewhat, in Park or Neutral, in your driveway.
If you connect a source of vacuum to the EGR valve, it should open the valve and hold vacuum. If it doesn't hold vacuum, the valve is defective. If you do this with the engine running, the idle will suffer, maybe even stall the engine just like when you lifted the pintle by hand.