I'm amazed that you were able to get a 9 speed quick link closed on an 8 speed chain. I once bought a 9 speed, KMC, "missing link" to join a Shimano 8 speed chain. I messed with it for an insufferably long time on a couple different occasions. Couldn't get both of the pins to get into the outer plate holes at the same time so I could then snap them home.
Finally, I sheepishly returned to the LBS and learned that I had purchased a 9 speed quick link for my 8 speed chain. To address your situation, if you've got your 9 speed quick link closed on the 8 speed chain (BOTH pins engaged in their home/closed position) that should be okay. Getting it to unlock may be quite a feat as it must be incredibly tight.
Regarding your comment, "I don't want to drive out a normal pin, and drive it back in - that's let me down before." A Shimano 8, 9 or 10 speed chain requires (according to Shimano) a dedicated, special link pin to rejoin the chain when a regular pin has been driven out. You most definitely will soon have a broken chain if you try and reuse the regular pin you've driven out. A chain joined that way will come apart there in short order. The special chain connecting pin specific to that speed chain must be used. If these are installed correctly, the chain will be very robust. Also of note here, despite Shimano's warnings against the use of third party quick links to join their 8, 9 & 10 speed chains, there is abundant anecdotal evidence that these work just fine and the practice is very common in the wild. KMC's missing link and SRAM's Powerlink are the two most used quick link products to accomplish this.
I suggest two things: first, examine the current connection of the 9 speed quick link (QL) on your 8 speed chain. Make sure the outer aspects of the pinsa are, in fact, fully engaged in the locked portion of the outer plates of the QL. Wouldn't surprise me if one side wasn't truly engaged and that's why it even fits at all. Second, get the appropriate speed of QL for your chain. Inexpensive peace of mind. Or, insert the appropriate chain connecting pin. In any case, get to a more permanent--and safe, and reliable--solution as soon as possible.