Is there an on-chain way to do an aggregation of an item that actually is the same thing? Eg, suppose I really want to see how much /actual/ ETH or USD is on the market across all issuers? Or is this necessarily foolhardy because we must always consider the issuer’s credit? If the latter, are there systems in practice for pooling credit together to make the trade require less due diligence?

What did you end up using?

Hmm, OK. So, AlsoSteve can issue SteveCoins, but if people buy them, wouldn't this create an impression RealSteve is obliged to redeem them for Steve-High-Fives? He is not, but if you bought them from AlsoSteve, you might be very upset with Steve if he doesn't honor AlsoSteve's promise, right? How do you prove, "I'm the /Real/ Steve, these are the only ones redeemable /from me/," even though it is super valuable that people can make coins redeemable from AlsoSteve/themselves?

"amount" is the cumulative amount in circulation of the asset across all issuers, or the cumulative amount of the the asset_code+asset_issuer combo?