The nonce is signed as one of the tx members, otherwise Ethereum couldn't distinguish between two legitimate tx's with exactly the same arguments / contract / method call, versus a duplicate sent accidentally with a network retry.
Your front end doesn't need to wait for each tx to be mined before sending the next one. That would indeed be slow. If you ...
This is probably a bad idea.
First, the "pending" transactions are subjective from the perspective of each node. They can only report transactions they know about.
You have seen that yourself.
Testing in the Kovan network may include or exclude pending as follows
Second, pending transactions are not reliably confirmed. They can be cancelled by the ...