The answers depend very much on which U-boat type and when. Snorkeling was a new procedure and the learning curve was steep, so lots of progress was made in a short time. There were at least 3 snorkel head designs starting with the original ring valve type which was replaced by the ball valve and finally the best one, the T-valve. The T-valve, used on the Type XXIII, was virtually invisible even to 3 cm radar.

Radar detectors were often used on snorkel heads, but they tended to lag behind in their frequency range, so when the 3 cm radar came out, it took some time to get a 3 cm radar detector.

Nonreflective coatings were tried, but they did not work .

The efficiency of sonar while on Diesel varied with the sonar set and the angles of the signal. The balcony sonar late in the war would work quite well toward the front of the boat, but the noise from the engine would wipe out the effectiveness toward the back.

A large deployment of the XXI would have completely upset Allied ASW and led to high shipping losses. The days of just flying up and down the ocean with the radar on and sinkinging unsuspecting U-boats were over. Radar would have been virtually useless so just finding the XXI would have been rather difficult. It would have turned into a sonar war in which finding and sinking an XXI would have been a major production. A scared up XXI could go down below the layer at 200 meters, stay there for up to a week and then stick its snorkel up as far as 350 miles away.

Regards,
SuperKraut