As to why Holly kept going, I think the following might be relevant:
Initially having an intelligence quotient (IQ) of 6,000, Holly becomes
very strange after Red Dwarf has drifted through deep space for three
million years. An early demonstration is his tendency to completely
speak his mind, as when he points out the age difference between Dave
Lister and the pile of dust that was Kochanski, admitting that he no
longer had any social mores and that he'd gone a "bit perculiar". (RD:
The End) One of Holly's odd activities alone was the invention of Hol
Rock, a form of decimalized music with two extra notes that Holly
invented, "H" and "J".(RD: Kryten) He also has a collection of singing
potatoes to keep himself sane. (RD: Queeg)
This is relevant, because it is plausible that this senility affected his (it's?) ability to turn around; a plausible plan may have been to fly out for 1.5 million years, then turn around and head back, arriving back at earth just as the radiation returned to normal levels. –