If all HBD exceeds 10% of the value of all HIVE then HBD will lose some of its backing and could (probably would) drop below $1 in value until the amount of HBD was reduced or the value of HIVE went up.

Right now it is closer to 2%, so not a concern short term, but if the value of HIVE were to drop a lot and/or the amount of HBD being held were to increase a lot (without HIVE going up in value), then it could be.

There is a plan to increase the limit in an upcoming hard fork (to I think 20% or 30%, I'm not sure which), so the limit may become even less likely to be hit, but that change is not certain to happen.

No. It's the fact that if the price of HIVE goes way down to the point where the debt ratio between HIVE and HBD is below the haircut rule, the you no longer get 1 dollar worth of HIVE from your HBD when you convert it on-chain, you get less (proportionately to how much lower HIVE is in value). That means you could risk seeing your HBD fall down to something like $0.75, which would obviously ruin your gains if you sell at that price.

Thankfully, the current value of HIVE is quite a lot higher than where this becomes an issue, but that may not always be the case if we go into a harsh enough bear market.