To your last point:
Luca was "special" because, as the novel says, "He could do a job of murder all by himself." So Vito restricted access to Luca to himself only--not even Tom conveyed orders from Vito to Luca. He wouldn't want his capos to be close to Luca because he wanted the violent man's loyalty for himself, exclusively--it was the only way to safely handle the "dynamite." Sonny was trying to contact him as acting Don, otherwise Sonny probably had no relationship with Luca.