So from what I can gather, someone's leeching all the spare prana because conspiracies, but the grail didn't have enough enough leftover to make an eight servant, despite that possibly being precisely what the grail needs to stop these conspiracies.
Am I correct?
Also, Solomon can't be the only one who willed himself to be summoned. Heroic Spirits are by their very nature exceptional individuals, and the vast majority of candidates would possess a strong desire to return to the human world, whether it's to experience battle again, see what's become of their home in the past 2000 years, etc.
I also feel like it's be weird if the Second Owner could stop the Grail or a Heroic Spirit from leeching prana from Fuyuki's leylines. In general, wouldn't she be allowing that to happen anyways, since servants use quite a bit of mana/prana to power NPs?
No.
Sure, it's possible, but like I said, if you're doing that then you're broken enough to deserve a spot on the banlist.
There's a difference between using the city's mana and fully draining the leylines. That's like the difference between breathing in the carbon dioxide someone breathes out versus tearing out their lungs. Looking after the Spiritual Land is the role of the Second Owner in the first place, on top of this specific Second Owner being the one in control of the Grail System in its entirety.