NSEU
I did work the PW4000 SCU autostart, but it's been over 25 years and the memory is a bit fuzzy of the specifics. And since I recently retired, I no longer have the option of checking my documentation to make sure (they made me leave all that stuff behind
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But what I remember is that for the PW4000 if there was no light after the dual ignition, it would cut fuel, motor for 30 seconds, cut the starter and put up the AUTOSTART EICAS message. You are correct that in-flight there is no limit, it'll keep trying indefinitely.
There is a dramatic lack of consistency in autostart between engine types on the 747-400. It was the first time we'd done autostart at Boeing and we sort of made it up as we went. Originally, Pratt didn't believe there was any reason or advantage to having autostart and didn't want anything to do with it, while GE made it basic on the CF6-80C2 FADEC. When Pratt realized that the lack of autostart put them at a serious disadvantage, they had to develop the SCU (Rolls ended up somewhere between Pratt and GE - it didn't take an extra box like the Pratt SCU, but it was still a bit of a last minute add-on). As a result there wasn't as much consistency between engine types as there is on later installations such as the 777.