The potential for a ratio of 125% existed. How could a backing coin hold if the market cap of what it was supporting was worth more? The answer is obvious as we found out.

And chance for that still exist on Hive. You can't really compare both or write about HBD haircut ignoring value drop on "collateral". You just described one side of the deal, pretending it's all safer than UST, and ignoring main risk.

This 30% can be 125% very fast. Someone could say there is no limit for that. Even if you stop printing hbd, hive value still can go down.

Which factor have higher impact on mentioned ratio, hive price or people?
I think however you approach this matter price change is more important.
This might be even measurable, you can compare activity based changes on ratio to price based changes to ratio in time. Like: take ratio some time ago, could be date of mentioned change, compare it to actual ratio after time, and compare influence on change by activity -> printing hbd, burning hbd to change caused by hive price.

If we reach 30% ratio at 3$ Hive and hive price will drop overnight to 1$ it's not 30% anymore, right? You can imagine all the events following this scenario, printing hive to drop price even more to print more hive, etc...

So, Hive price amplitude is probably most important factor on the topic you describe, but you missed it. Possible any number above 10% creates risk, not necessary we will see it now at low hive price, but eventually with next bull period if people start cashing out to HBD and we will get closer to 30%, eventually price will drop and some funny events may occur.

Take a look at Hive chart, think about something like max possible value drop on Hive. Maybe 3 - 0.3?, 10x ? Then simulate how this will affect ratio at 30% :)

Worth to mention is that I know nothing about hbd itself. Wrote it based on your post and basic math knowledge.