No. I Got REALLY excited there for minute because I thought you had the elusive DE-9 M-M modem cable with the square connectors. But you are close. The Mini DIN-8 peripheral adapter cable you are using is a good start, but the Imagewriter cable will only work with a printer. Note the DE-25 connector is for use on the Imagewriter, NOT the Mac Plus SCSI bus! DO NOT plug this into the DB-25 SCSI port!!!!
What you need now is a DE-9 M-M modem cable, or a gender changer to convert your DE-9 Female connector of the peripheral adapter cable to a DE-9 male connector that will go right onto your Mac 128K/512K. That's a fairly common adapter you can find at almost any computer store as it is a common problem among PCs.
OH I FORGOT, for your Mac Plus setup none of those cables will work you need a Mini DIN-8 M-M cable, a very easy to find common Imagewriter II printer cable. Alternately you COULD use the DE-9 F to Mini DIN-8 M peripheral adapter cable, but you would also need a Mini DIN-8 M to DE-9 M peripheral cable. Though the latter is going to be harder to find than a normal Imagewriter II printer cable, which is all you need.
Basically there were two of these DE-9 converters Apple offered. One was used to adapt the existing proprietary Macintosh cables (like the modem cable) to the Macintosh Plus, hence the Female connector (and shortness). The other was designed to simply use a peripheral with Mac Plus that already had a Female DE-9 connector , so it was Male instead (and normal length). The two could be used together to create a Mini DIN-8 M-M cable.
(I assume you got the Keyspan model with two Mini DIN-8 ports?)
You're almost there!