Alert history it's a total diferent thing than Stream and ValueStream storage.
What you stand here won't be a problem for Alert History, alerts are thrown at Property Level not at Thing Level, than por the same thing, same timestamp you already can have more than one alert.
No it isn't. According to ThingWorx help:
In PostgreSQL and H2, stream entries are keyed by the unique key entries of the timestamp and source.
For AlertHistoryStream the source corresponds to thing name, and timestamp resolution is 1ms. So if you have multiple alerts for the same thing firing at the same time, then you may lose some part of the history in random order. We actually encountered this in the past, and it's easily reproduceable. Try it! On my laptop the latest 8.2.5 on PostgreSQL loses about 2% of alert history when I'm doing something as simple as setting/resetting alerts on the same thing in the loop. The ironic bit here is that the faster your hardware, the more data you may lose.
Well actually I don't know, I'm sorry, we don't use ThingWorx Alert system we have our own, was my fault to state something that I didn't tried.
Better you go and test what it actually does.