First of all I have to again pass the buck and say that the best tutorial on how to use the sonar in battle is by WernerSobe in his Sonar Only Manual Targeting Tutorial video
http://files.filefront.com/WernerSob.../fileinfo.html. This thing is so good! I absolutely guarantee the most fun you can have in SH4 is to smack the side of a target never seen with two torpedoes. Werner's tutorial answers all your questions, I think, about the actual operation of active sonar. The buttons I use for send to TDC are on the sonar button bar right beside each other, but you can use the toggles in the conning tower as well. I just like the two buttons side by side.
As far as maximum range from passive sonar goes, about 70 feet, 10 feet below periscope depth seems to be the sweet spot, but periscope depth is pretty darn good. I usually come to full stop at 70' if I want a comprehensive sonar sweep. Keep in mind that a functioning radar will do just about as well and give you more information besides.
As far as radar is concerned, I don't use the PPI display at all and just check out how it is plotted on the nav map. This is not unrealistic because that is exactly what a plotting crew would do. A sub captain didn't plot his own targets, although he might back up the TDC with a banjo to keep it honest. Usually he had another officer do that too.