Meh, I think you're fine for the most most part in my opinion. Like I said, none of what you described is outlandish, just rarely seen. In the case of the first Joker, we've had cases of Phantasmal Beasts and/or their remainders remaining in this world so the possession of a feather wouldn't be impossible, but it goes without saying that it is a priceless artifact. Though in the case of Servants... saving a human who is near-death I can see, but I'm not all that sure how much such healing abilities would help when faced with the fractured Spiritual Core of a Servant. Seeing as they're not beings of flesh and blood, they're entire existences are built upon the Spiritual Cores that are connected to their head and heart, so I doubt they could recover from such injuries. Any other injuries should be easily healed by an artifact of such power however ^^

In regards to the second Joker, I'm assuming the ability to temporarily disable Magic Circuits is less of an alchemic poison like the one used by the members of Zugzwang, and more of an ability to force them to "shut down" similar to a computer and forcing them to "reboot", hence explaining the few hours needed for the victim's Circuits to become operational again. They've been describes as both mechanical and organic several times in the series, so causing them to behave this way isn't too far-fetched, but the question is more of a "how" rather than a "is it possible". You mentioned that the first Joker was crafted from a Phoenix Feather, so mayhaps the second Joker could have been crafted from materials harvested from a Succubus, Alp (from German folklore), or Kanashibari (from Japanese folklore), all of which are known to be associated with sleep paralysis or other inhibiting phenomena of some kind. The fact that the card contains traces of these Phantasmal Beasts would explain its powerful effects, seeing as you mentioned it even affects Servants to some degree, and since you've already gotten ahold of a Phoenix Feather, one more monster part wouldn't hurt, eh? xD