AR-9 saga continues.
After checking with other people who know more about guns than me and confirming there is no decent way to mate the Spike's Tactical Lower with the Kaw Valley Precision upper, I have decided to get rid of the lower, get a Glock magazine adapter for regular AR lowers which I know mate well with the KVP. The other option would be an even swap for a PSA dedicated lower locally as I need to make 100% sure they do mate properly even though I keep getting assured over a stack of bibles they do indeed match.
And if I get any more annoyed, I will be willing to trade the whole shit for the shortest complete and reliable 5.56 upper.
A Ruger Charger in 9mm will also be very acceptable. And I have the nagging suspicion I should have gone this way from the beginning but wanted something super light for the missus.
One thing is for sure: This is my last AR-9 build. At least till they get their shit together and standardize the whole platform