I believe Mainline comics was connected to Prize comics.
Yes. Crestwood/Feature Publications, for whom Simon and Kirby drew, wrote, and packaged comic book lines, previous to their forming their own "Mainline Comics", and who also published their own Prize Comics and its related Prize Comics Group titles, also shared their office manager, Nevin Fidler, who was hired by Simon and Kirby to be their new publishing company, Mainline Comics' business manager. Mainline was a small, totally independent publisher, owned by Simon and Kirby and Fidler, but had Fidler as a go between between the two companies, and Simon and Kirby still worked for Crestwood and Feature on their Prize Group titles, separately from their own Mainline work. So, I understand why Mainline isn't stored on ACB+ together with The Prize Group, or Crestwood/Feature Publications. it should be in the "Small Press" section.
I wonder where "Mainline Comics" is located on CB+ and DCM? I can't find it. As it doesn't seem to be here, maybe it's still not PD? It IS listed in GCD under Mainline Publications, including their "Bulls Eye", "Foxhole", "In Love", and "Police Trap" Comics series, but not including "Mainline Comics", which I've seen at least one issue as a mid 1950s title.
