Karochii's (artist #38002) group is mochiya (餅屋, Mochi-ya). His old home page (now defunct) was at http://rinen.sakura.ne.jp/
There is a group artist entry named Mochiya (artist #4414). The two URLs are http://rinen.sakura.ne.jp/ and http://rinen.sakura.ne.jp/0416.jpg/. However, the only member of this group is the artist Mochiya Mochiko (artist #88089), who is banned. Every single one of his URLs is bad, and http://rinen.sakura.ne.jp has never been one of them.
Details
2009-03-15 18:42 Tag "mochiya" first applied (to post #421245)
2009-10-16 07:52 artist #4414 ("mochiya", actually a group) created. URL: http://rinen.sakura.ne.jp/
2009-10-16 08:10 artist #38002 ("standalone") created
2009-11-02 22:35 Artist "standalone" gets URL: http://rinen.sakura.ne.jp/
2010-08-24 13:23 Tag "standalone" is first applied (to post #733532)
2012-01-02 13:04 Artist "standalone" Group set to 餅屋
2012-01-16 05:36 Tag "mochiya" last applied (to post #1077113, later removed)
2012-08-03 14:27 Artist "standalone" Group changes to "mochiya"
2012-08-04 13:55 Artist "standalone" is renamed to "karochii_(mochiya)"
2012-08-04 13:55 Wiki "karochii_(mochiya)" created
2012-08-04 13:56 Tag "karochii_(mochiya)" is first applied (to post #525722)
2012-08-04 13:56 All instances of Tag "standalone" replaced with "karochii_(mochiya)
2012-11-27 00:20 Tag "mochiya_mochiko" is first applied (to post #1305211)
2012-11-27 00:42 artist #88089 ("mochiya_mochiko") created.
2013-05-02 12:53 Tag "mochiya_mochiko" is last applied (to post #1409447)
2014-08-29 13:23 Implication "mochiya_mochiko" -> banned_artist activated
2014-08-29 22:15 Wiki "mochiya_mochiko" created
2014-08-29 22:15 Artist "mochiya_mochiko" Group set to "mochiya"
2014-10-22 02:34 Artist "mochiya_mochiko" Group unset
2016-01-23 04:55 Artist "mochiya_mochiko" Group set to "mochiya"
2017-07-29 13:52 Tag "karochii" is first applied (to post #2797797)
2017-07-29 13:53 Artist "karochii_(mochiya)" is renamed to karochii
2017-07-29 13:53 Wiki "karochii_(mochiya)" is renamed to karochii
2017-07-29 13:53 All instances of Tag "karochii_(mochiya)" replaced with karochii
My conclusion is that setting mochiya_mochiko's group to mochiya when they were banned in 2014 was definitely wrong. However, there does not appear to be any way to undo it; editing mochiya does not yield any means of changing group members.
The best solution I can see is: