Thanks for writing this up. Having considered the overall scope of Dante's abilities, here's what we're going to do.

↠ The humanization process of arriving in Thedas means that Dante is physiologically limited to what would be possible for a normal human of his actual size and muscle mass--i.e., if he's not built like the World's Strongest Man, then he can't lift like the World's Strongest Man. He will still be able to reach peak human capacity like you describe, but it will now be because magical energy is being used, essentially as if he were using the Arcane Warrior Dragon Age specialization. This doesn't mean that he becomes a mage, and for the most part his stamina and strength and so on will function exactly as he's used to, but it does mean that the process that's going on behind the scenes requires magical energy that can be exhausted if used for extended periods without breaks, and anything that blocks or interferes with magic (Templar abilities, magebane, etc.), will reduce him to the level of a normal human of his size/build. This process isn't something he'd be aware of or able to impact except through the normal ways that humans experience and impact their own energy cycles (resting, eating, etc.). The limits of this energy are obviously tough to quantify, but somewhere between "going toe-to-toe with one qunari" and "going toe-to-toe with 10 qunari back to back." Our general rule of thumb is that if your character's abilities are single-handedly solving a plot, winning a battle, or otherwise making other PCs around him superfluous, then you're going too far.

↠ Your descriptions of what 'peak human capacity' will mean work for us except for speed. Fade Step is a magic level beyond peak human speed with an almost-teleporting quality to it, just over short distances. So his peak speed when not using Devil Trigger will be lower than that.

↠ When he is using the Devil Trigger ability, Dante will be able to double his strength as you describe and move at speeds equivalent to the use of Fade Step. He'll also be able to imbue his weapon with lightning/fire/ice magic equivalent to the application of a rune, and hover/glide but not actually fly. He'll be able to maintain this form for up to a minute at a time, and as you suggested, this will work similar to the Power of Blood ability and cause him pain/exhaustion that he'll need to recover from afterward (it will also impact his ability to use his peak human capacity afterward, since it's all drawing from the same well of magical energy). Otherwise your nerfed version of this ability as described above works for us.

FYI, because his Devil Trigger form resembles Thedas demons, using this ability in front of people outside Riftwatch could have negative repercussions on Dante individually and on Riftwatch's reputation. (It could also of course make a negative impression on people within Riftwatch, but on the whole they tend to be more accepting of rifter strangeness.) If you plan to have him transform in a public setting, please either be prepared for negative IC consequences or consult with us first.

↠ His sword can transform into a spear, but its other abilities will be eliminated. Transforming into a spear won't pull Dante along behind it or anything like that.

Let us know if you have any questions. Otherwise, you're free to go ahead and RP!