Are there any reason to send radio reports?
I can not 100% confirm this but , it seems to me that sending radio report will slightly increase chances of useful contact report reception in your area.
For example if I see a merchant or a warship, is it any good reason to report that back to base with the radio?
Also after I sunk a ship, should I report that every time?
In this situation, you should use "send contact report" button...Later in the war, this report will have random chance of spawning U-boat wolfpack in your area so you could attack the contact together. Be warn that AI subs are not perfect but if nothing else they can distract escorts
...(if you use TWoS or TDW New IUs)
Will other allied uboats, warships, airplanes that are in reach go after the ships I report?
I'm not sure but I think the answer is no, although I would be happy if someone could refute my claim...
Can I radio unarmed merchants to abandon ship before I sink them?
Hehe, no, unfortunately...