SQL server restore readonly file group without backup from .ndf file
note that when you ran the RESTORE FILELISTONLY look at the column IsPresent and for the readonly file it shows 0 
this mean that this file does not exists in the backupset.

anyway this file cannot be restored from that backup set because it does not exists there, so you must take it from a full backup (or just a file backup) 

the only option to reset the "Archive_Pending" is by restore that element (either file group, file or page) and make it consistent with the database LSN (log sequence number)

the term attache is only for new attached databases and cannot be used with files.

so you can take it from full backup or from file backup (any backup that this file is "IsPresent" = 1 ) 

to continue your DEMO SCRIPT

-- take backup of that RO file group
BACKUP DATABASE [TestDB] 
FILEGROUP = N'TestDB_Part2' 
TO  DISK = N'D:\data\File2Backup.bak' ;
GO


-- restore the RO file / FG
RESTORE DATABASE [TestDB2] FILE = N'TestDB_Part2' FROM  DISK =     N'D:\Data\File2Backup.bak' WITH  FILE = 1,  NOUNLOAD,  STATS = 10
GO


and thats it.... your DB is up and running

I hope this has been informative for you, please let me know if you have any more questions.