[UnderseaLcpl]

1.) Not in vanilla sh3 sadly. I guess you could draw letters with the ruler but thats about it. As far as I know, you don't get that in GWX either.

2) Not really. GWX does include a nice slide-out compass rose in the lower-right hand corner of the screen that does make entering a bearing a lot more precise.

3) I have no idea. Frankly I have better luck using deck gun from the "normal" view and guessing/looking for impact flashes. Also, you could just let your gun crew fire. With a good watch officer who has the gunnery skill they aren't bad.

I'm not sure if insane range periscope lock works in GWX, haven't tried it. Yes, destroyers and many other vessels can accelerate at absurd speeds. GWX doesn't really fix this. Best bet is to use an electric to torpedo them or take their inevitable spotting of the torpedo into account.

DD's will identify and pursue you from a pretty good ways if you shoot the deck gun at them. I don't know about 20000m but in GWX this won't really work. As soon as you sink one DD the rest will converge on you. As well as other sub-hunters within, like, 50 miles, and airplanes, and probably sharks, and Winston Churchill.

Airplanes. Oh you ain't seen nothing yet. In GWX sh3 stops being a subsim in late 41 or so and becomes an aerial bombing victim sim.
Your only hope is to use the latest radar detector, only surface to charge the batteries or in safe waters, and always cruise at 30m or more to keep them from spotting you and bringing destroyers to the scene. Incredibly frustrating, and incredibly realistic.

Hope that helps.

edit- Almost forgot, welcome aboard and happy hunting!