Thanks for your feedback!
Here is my commentary.

HBD [...] is set to lose its peg by design. This is what we refer to as the Haircut rule.

This is not what we refer to when we talk about the "haircut rule"!

The haircut rule is the fact that when the debt ratio is higher than predefined thresholds, the blockchain will slow down or even stop printing new HBDs for some of its HBD creation mechanisms.

This is mostly wording, not sure is there is a definition about what exactly is the haircut rule. For the regular user the important thing is that HBD is no longer one dollar, and this is what I have been referring here.

Referring to the overall process, stop printing HBD and then lose the peg is a good point.

When the debt limit is reached the blockchain no longer guaranties a one dollar HBD. The highest the debt the lowest the price for HBD from the blockchain.

Again, this is not correct. The debt limit has no direct influence on the price, but only on the printing of new HBD. The price is driven by market forces, not by the blockchain. The HBD might lose its peg, which is very likely, but it might not.

I have pointed out in a several places and it the cited sentence that we are talking about the on chain HBD price, not the market price. The price that the blockchain values HBD when converting to HIVE. The market price can be totally different, in fact this is now the case for SBD, where the debt limit is reached but the speculators keeps the prices above one dollar. But I have made a clear distinction between these two type of prices.

First let’s take a look at all the ways HBD is printed:

• Author rewards
• Conversions
• Interest
• DHF payouts

There are 5 mechanisms involved in the creation of HBD:

payout of author rewards
conversion from HIVE to HBD
interest payment on HBD stored in savings
allocation of part of the inflation to DHF
conversion from the ninja-mined HIVE

For this part I think its important to distinguish the total HBD supply and the available (?) HBD supply. What I mean by this, is will we include, or exclude the HBD in the DHF. For the debt calculations the HBD in the DHF is excluded, and because of this I have taken a look at the available HBD. If we look how HBD enters circulation as available HBD, then its the points I have mentioned, while if we look how HBD enters the overall circulation then it is the points you have mentioned. But for the debt calculation the HBD supply that counts is the available HBD.

Author rewards
At 20% the HBD author rewards stop being printed and all the rewards are paid in HIVE only.

This is not correct. If the debt ratio reaches 20%, the blockchain will start to slow down HBD printing as defined in the blockchain code (database.cpp / line 5370). Authors will be paid with both HBD and HIVE. It's when the debt ratio reached 30% that the payouts will be HIVE only. If this were not the case, there would be no point in having two thresholds.

This was one of the surprises for me, but its true. There in lower and upper soft limit. The upper soft limit is now taken down to the lower soft limit, but the hard limit remains. In short the point I have made is valid, there is no transition period in which authors receive, both HBD and liquid HIVE as rewards, but it sharply switch to liquid HIVE only. I have consulted @andablackwidow for this and he has pointed out this is the case.

DHF Payouts
Another source for HBD are the payouts to proposals.

This is not correct!
The haircut rule has no influence on the payment of DHF proposals for the simple reason that it is not at that time that new HBDs are printed.

It is the provisioning of the DHF which is the source of creation of these new HBDs. Even when no proposal would be supported by the community, meaning no proposal payouts, the blockchain would continue to create new HBDs and add them to the overall budget. There is no way stakeholders can influence this dynamically.

It should also be noted that the DHF payouts are not part of the HBD supply. They are just HBD transfers from @hive.fund to the proposal recipients.

Here again we come to the point in the two different types of HBD supply. Total and available. The available supply is what counts for the debt (excluding the HBD in the DHF), and when DHF payouts are made, that increases the available supply and the debt levels.