I wonder, would it be possible for a competing miner to capture these blocks and insert their own (valid) transactions in place for broadcast?
No because the block has to be changed and if you change even a single bit the final hash will be entirely different meaning it will no longer satisfy the target check so it has to be mined again.
As I was typing that, I was thinking that was the case, but it's been quite some time since I've dove into the technical details of bitcoin mining. Thanks for clarifying.
What would be the goal or take away from a malicious node broadcasting these intentionally?
I don't think this is intentional. To give a better answer we have to know the client the said node is running which may be acquired if your logs stored the IP address of the node that sent the invalid block. In which case we may be able to connect to that node and see if it sends its useragent and that may give us more information like the client name which we could find on GitHub and report this bug.
While I do have logips set to 1, everything I can see in the debug.log is quoted above. Is there somewhere else I can pull the IP of the invalid broadcaster from?