Q1: I have tried to reduce the number of calculations. Instead, I use comparisons. Also, I like when crits and fumbles happen often.
So crits works a bit like Unknown Armies: if ones and tens on the die roll are equal (as in 11, 22, 33 etc), the roll is a crit. A critical success means that something nice happens, a critical failure means something bad happens. No special rolls – I didn't think they added enough to the experience to motivate additional complexity.
Opposed skill rolls is similar to Pendragon: you roll under your score, but over your opponent's roll. I love the elegance of the Pendragon opposed roll.
Q2: There are hit points. They're just not depending on SIZ+CON.
Instead, there's a tiered hit point system akin to Wounds/Vitality in Star Wars d20. There are two hit point pools, Guard and Health. Guard measures energy, momentum, defensive posture, balance, breath etc, and Health is, well, your health. Each race have a base value representing an average adult civilian without any combat experience, but you can increase them by allocating dice to them in character creation.
An ordinary hit reduces Guard, and if Guard runs out, the rest reduces Health. An exceptional hit reduces Health directly, bypassing Guard. Guard is quickly restored: take a moment to catch your breath, or get support from a friend, and Guard goes up a bit. Guard is completely restored when the scene is over. Health heals slower: even with care, it could take weeks to recover from serious injury.
Balancing wound systems is tricky. Wound systems tend to make wounds either inconsequential in the long term (either naturally or by magic inflation), or halting the story or removing the character from the story while recovering. I think I have found a nice balance with Guard/Health: you're capable of participating even after being hurt, but you're still hurt. And I think that it's way cooler to be hurt and still struggle on than to be magically completely restored.
My experience is that Järn is less deadly for unarmoured characters compared to BRP, and more deadly for armoured characters. Also, fights between very two experienced fighters, or between two noobs, tends to resolve faster because there are fewer rounds where nothing happens (neither fighter hitting the other, or the other fighter always parrying) because of the Pendragon-style opposed rolls. For matched armoured reasonably experienced fighters, I think fights are resolved at the same speed as in regular BRP.