1: I would use a high carbon steel, the tool steel might be a bit brittle. Brownells sells some stock for making firing pins from scratch, "La salle fatigue proof steel"
2: Measure the force of the original STEN spring (compress it on a scale) and start there. Since you are taking weight from the bolt I would start out a bit stronger. A rubber buffer might be in order as well, most of the modern blowbacks use them.
3: Threaded end cap? Lots of subguns use that type, many with interrupted "cannon breech" threads so a partial turn releases it. Or just use the original STEN parts. Cut the washer shaped section off the stock and use it as a retainer plate inside the original rear housing.