Two comments. For balance, one on each side of the debate. :)
Don't forget, the dying spirit has to be NEARBY for that top charm, and WILLING to die to heal the Sidereal. That makes it a bit more difficult, on top of the afore-mentioned difficulties.
More precisely, the Charm leads you to the nearest spirit, of the relevant sort, on the verge of death. I think it's reasonable to assume that you get there before it dies, since this is a fate manipulation, unless creatures outside of fate muddy the picture. But it could still be thousands of miles away, or ten miles away and surrounded by drunk Abyssals. That's why some Sidereals "arrange" for a spirit to be near death before using the Charm—-but, of course, that makes the spirit more likely to refuse the bargain or become an enemy afterwards.
"Sidereal's cannot make up new Charms. Solar Exalted can." True, but anyone who wants anouther homebrew charm can simply add it and say "it has always been there." Hell, even the person who wrote the charms spoke of their being lost and hidden ones. The lack of ability to make up new charms is meaningless in a practical sense.
Well, sure, but if you ignore the intention for how Charms work, then Dragon-Blooded can make the Reflexive Ess1/Perf1 1 mote Charm "I Win", and the whole debate over Sidereals is pointless given how obscenely abusive the D-Bs, Eclipses, and Moonshadows are. :)
Can a Sidereal use Earth and Sky Bargain multiple times to play host to multiple spirits concurrently?
Sure, I guess. It doesn't say it's cumulative, so if you want to pay 10m, 1W, 1h for another 3 soak, go for it. :)