Sorry Harry, Heath is just up the road from me and we joke a bit.
Heath, definitely do a graft. With a really sharp stanley knife (nice sharp point) slice in and cut the yarn through the primary backing at each end clear of the pull so that you can pull the loose yarn out and leave a nice tight end on there. With a nice sharp pair of duck bills, cut out the remaining primary backing and scratch out any blobs of latex that will hold your donor up.
Determine the nap of both the carpet and the donor, and MARK IT on the back of your donor. Now cut along and cut off the necessary number of rows by slightly longer than the section to repair.
Attempt to lay it in there and see how high it sits. If it sits right height wise, leave it at that, if it's sitting high, attempt to pull the secondary backing off. It'll be stronger if you can leave it there but no use having it sitting high so if in doubt, I'll rip it off. Take care and you should be fine.
I then usually start at the end where the existing nap is laying over into your repair area and work away from that point. With your glue gun really HOT (don't be tempted to start till it's hot as the glue will cool too quickly before you can do the necessary manipulation of the donor.) Only glue a small section to start with, maybe up to an inch and then bed the donor in and make sure you're happy with the result before you carry on.
Depending on how long the donor piece is, I tend to wait till I'm within an inch of the end before I trim the total length. So just progress along at small sections at a time.
Having an awl is great too because it really helps in bedding those edges down into the glue.
See how you get on and let us know. With time on your side, simulate such a repair on an offcut and see how you go first.
Retufting is NOT an option where you don't have a complete backing in place or over jointing tape. Because the damage is along a seam, the original problem is that they've cut too close to the base of the tufts AND not seam sealed so you don't have enough material to tuft effectively into. (I'm sure there will be guys out there that do, so this is purely my observation)
John