Honestly, and I know this isn't the generally accepted take, but - this manga didn't make me give a shit about Arika. She didn't show up, we didn't get to know her - I would much rather have an ending where any one of the main cast got a happy ending than the useless symbol Arika (and that goes partly for her fans - can't sympathize with the fans of a cardboard cutout). The shitty moralizing / fantasizing about drugs didn't help, but the main problem was the lack of characterization for Arika. I didn't dislike her in any way, it just felt absurd that the manga would think I cared what happened to her when she was a complete non-entity...
You pretty much described what a mcguffin is, a plot device meant to drive characters but is generally unimportant overall. We didnt need to care for Arika (at least not directly), we needed to care about the characters who care about her. Not to say the author didnt try to characterize Arika, but any characterization she was given served as more motivation for Nana.
The early parts of the manga made Arika more interesting with how she was this haunting figure over all the characters. But the more we knew about her, the less impact she has (mostly because the list of suspects lessens for every new reveal) until there was only one person we care about caring about Arika, Nana.
Because of how indirect being invested in Arika was, whether you care about her or not really depends. I care about Arika, but mostly cause I want Nana to be happy. Hopefully some extra chapters pop up.
As for Arika's fan? Yeah... The fact they barely showed up near the end, and this chapter focusing more on them being Nana's friends than Arika's fan (Arika being a reason they became friends) just adds more to Arika being a very good mcguffin.