So I have an idea of how Donald Love's business relationship with Valiant Video Enterprises and The Pickman Project:
(Keep in mind that this is only a headcanon/theory, so please take it with a massive grain of salt)
Because Donald Love was able to get away scot free in the end of both GTA 3 and Liberty City Stories, it would make sense for me to believe that he had connections from all over the country.
Therefore, I'd like to think he left to Carcer City after the events of LCS to make business deals with Nr Nasty and Lionel Starkweather. Given how he enjoys his "morgue parties", I'd say it's canon.
As for his disappearance in GTA 3, my take is that he went back and forth between Carcer City and Cottonmouth for his special deals with Mr Nasty, Starkweather and Pickman.
In 2001, after the Pickman Project was abandoned by the government, Donald Love would become one of Pickman's private funders. And after the events of Manhunt 1, Donald Love would cut all ties with Mr. Nasty and covered his own tracks, prioritizing The Project.
Due to the condition of Cottonmouth during this time period, Love would keep a necessary distance while maintaining the business relationship with Pickman until 2007.