Well, you are indeed entitled to recieve an award for resourcefulness and ambition.
1)You answered one question I was actually thinking about today---are manhole covers grey cast iron or ductile? I think from your first picture you cut most of the way through the casting and then broke it the rest of the way. That is perfectly reasonable and understandable. The key finding is that the crystal pattern of the iron looks to be grey iron. That is good as it may scrape a little easier than if it were ductile. Had it been ductile you would have had a good deal of trouble breaking it.
2)I would strongly encourage you to thermally stress relieve the section you intend to use as a straight edge. I say this as the only scientifically proven method (there are numerous anecdotal reports about other methods with often conflicting results and even disagreement among observers of the same "demonstration." So beware of other methods unless you can find equally strong scientific evidence---as long as you think science should answer such questions). I think it is safe to say that thermal stress relief unequivocally does just that. All you need to do is find a large electonically controlled pottery kiln. Put it in there, raise the temp maybe 300 degrees per hour until you get to 1150. Hold it there for 90 to 120 min and then let the kiln cool slowly over 8 to 12 hours. That will do the trick as proven by the US Navy in 1948. I'll bet your school owns a large enough kiln. Otherwise, if there is a foundry nearby, you may be able to get them to heat treat the piece for 10 bucks or so when they throw it in with their next grey iron stress relief load.
https://apps.dtic.mil/sti/pdfs/AD0620556.pdf
3) As a small-foundry caster of grey iron straight edges I can say the "inclusion" is not an inclusion. It is a vacuum defect caused by the molten metal shrinking as the casting cooled. There was no riser at that point (a very thick segment in the middle of the casting) to feed metal into the casting at that critical point. So, the metal shrinkage was so strong that it simply did the only thing it could and formed a vacuum defect in the center of the casting. This occurence is by no means rare. I have had to learn how to prevent it in my own castings and I know of multi-ton casting used in bridge construction that suffered internal voids for the same reason. As others have said, it probably is not structuraly significant. Do not try to fill it in by welding the casting. This can be done by an expert (true epert cast iron welders are very rare), but it is more likely to be screwed up by someone who "knows how to do it." Attempts at welding are likely to really mess up your cast iron. Don't do it.
Denis
Added: That manhole cover is very likely to be full of stress. The goal of the manufacturer was to make a structurally sound crude casting. He did. But likely it was removed from the mold just as soon as it was solid. No problem for a manhole cover, but not at all right for a straight edge. I am very careful to let mine cool to 150 degrees or less in the sand. Then I pull them out and then THERMALLY STRESS RELIEVE every one. Doing so will allow that retained stress to relax and all will be well.