#1 Try to release when your hand is around the 4:30 position on the disc. Nose-down putts come from a 6:00 release point.
Figure out whatever it takes to get a perfectly flat, wobble-free flight that has some loft to it. Disc angles can come later.
Instead of trying to speed up your arm for more distance, add more weight shift. At the distances where a push/pitch putt is useful, arm speed isn't really a factor. Keep the arm fluid and pay more attention to how body weight + finger spring can make the disc fly.
#2 At the elbow end, the forearm angle should be more like holding a glass of water, but at the wrist end it will be pointing more at the sky (so, some water would pour out the side).
The point of the finger spring is not really to propel the disc, it's to get the fingers off the disc cleanly while controlling the angle. Also I think "finger spring" is sort of a tricky way to get the wrist opening naturally, without over-muscling it. For me, missing right usually means I yanked the wrist. But if I pay attention to "flinging the water" toward the basket, the wrist tends to behave.
That grip sounds more like what you'd use for a spin putt. For a pitch putt, try this:
1. Place putter in the seam of your hand, tucked under the base of the thumb.
2. Place ring and middle finger under the disc in a position that allows you to support the disc flat, without the pinkie, index finger, or thumbtip touching the disc. You've got the base of the thumb on top, and the two supporting points below.
3. Gently grip the rim of the disc between the thumbtip and index finger. Longer putts need a firmer grip.
4. Hold the disc like this in front of your belly button. The disc should be perfectly level, and still mainly supported by the three points from step 2. The base of the thumb should be around 2:00, and the thumbnail will be pointing off to the left.
5. Now lift the disc up to about chin level, and forward a bit -- whatever is comfortable. Be sure to keep it flat! Play with different heights, different amounts of wrist opening, etc., always keeping the disc level. For the actual throw, you'll be lifting it a bit and moving it forward a bit, and the thumbnail will end up pointing directly at the basket. Rehearse this in slow motion, allowing yourself to feel the weight of the disc and how it moves.
Now practice short putts, 10' - 12' or so. The goal is to pop the disc toward the basket, lofting it up to maybe eye level, and keeping it flat for the whole flight. It is possible to do this with very little arm movement and a slight hip-thrust. Try to release the disc close to the body, with a bent elbow.