Maybe somehow the opening of these 10-15 different wallet files has messed up the entire Bitcoin Core client for me.
-snip-
Maybe.
There's a chance that Bitcoin Core failed to scan the correct balance and kept the spent inputs in your wallet.
I've seen a couple of topics regarding that, usually those who replaced the wallet.dat file.
BTW, what error did you get when you tried to broadcast the transaction(s) via console?
In case you missed it:
You can also try to manually broadcast it and see what mempool-related error you'll get:
- In the transactions tab, right-click on one of the transactions, then select: "Copy raw transaction"
- Open the console (Window->Console), then type: sendrawtransaction "Your_Copied_RAW_Transaction"
- And it should return with an error which should be the reason why those aren't successfully sent.
If it somehow proceed, then there could be in your setting that's preventing txn broadcast..