Heya! Just ran a one shot with what I assumed last minute and I got pretty close to what you have in your manuscript.
I ran this Re-Energizers - Catthulhu | DriveThruRPG.
HP
For this I just used the creatures straight up HD value as their HP. This produced what I felt were pretty squishy creatures, so I think you might be right with HD+5, or maybe HD+2
Damage
I think d6 is a bit high. I used d4 for most attacks and scaled it similarly in the same way. I decided to use critical hits as dealing max damage on a single die instead of doubling the dice, I think this is much more forgiving for both creatures and the players.
AP
I removed armor more or less on all creatures in the adventure. This may have contributed to them being a bit squishy.
The special effects and ability I never got a chance to use. So far so good however! Players loved the system! My only thought though is that the Follow Through trick is so extremely good, especially on a level 1 character. I'd probably limit it to a number of extra enemies equal to their Strength score, theres also no real idea to the limitations of Relentless other than the player being invincible, so I instead limited it by "You're up below 0 HP but if you go below your health in the negative you die instantly"