The Infinite Seal is not worth it
After about 5-6 hours of playing the Advanced Access, I've realized that the Infinite Seal is not at all worth it.
The enemies in the main game hit 2-3x as harder compared to the Demo, not to mention that there are multiple enemies who are harder in general owing to their variety of attacks.
While I loved using the Infinite Seal in the Demo, it's no longer a favorable risk-reward to use it in the main game. In the Demo, whiffing an Infinite Seal parry meant taking 10-20% of damage or 20-30% against tougher foes. In the main game, it's more like 50% even against some of the regular enemies and straight up 70-80% against the tougher ones. You'll be practically dead unless you are at full HP at all times. The Infinite Seal's parry simply takes too long to 'reload' once you whiff it.
The other problem is that the combat system is not at all made for taking on multiple enemies at once, and the Infinite Seal just makes it that much worse. I've multiple ocassions where I whiff a parry and receive attacks from two different enemies at the same time, effectively kicking me out of the shell. That rules out using the Infinite Seal for general exploration, which sucks.
Unless the 'reload' window for the Infinite Seal is reduced drastically, I see no point in using it. Once you whiff a parry, the Harbinger holds the knife high above his head. There should be an animation to bring it back down quickly to trigger another parry window. However, I don't know how realistic it is to expect the Devs to include an entirely new animation in the game post launch.
Possibly the only use for the Seal right now is in fights where you've memorized all the attack patterns of the enemy (I did use it sucessfully one time against >!the Grisha!<). Even then, you need to practice fighting the enemy a little more to adopt the parry window for the Seal. It is too much effort for little reward and a whole lot of risk.
I am personally alternating between the Untarnished Seal and the Vatra Seal, with a slight preference towards the Untarnished Seal. I think the parry window provided by the Untarnished Seal is generous for my taste, but at least it does not mean death when I whiff a parry one time.