It's worth noting that intentionally creating many low-fee or feeless transactions can contribute to network congestion and may be seen as spam behavior, which is generally discouraged by the Bitcoin community. As such, it's essential to use the Bitcoin network responsibly and pay appropriate fees for your transactions based on the current network conditions.
It doesn't congest the network in any meaningful way. If you attempt to broadcast a transaction with zero fee or a fee below the dynamic lower limit set by individual node's mempool settings, then nodes will simply not relay that transaction. Repeatedly try to do this, and nodes will simply blacklist your IP and ignore you.
You can send with a 1 sat / vb fee which will probably never get confirmed at this point.
You can't really, at the moment. Given that the vast majority of nodes run with default settings, anything below around 2.9 sats/vbyte at the moment will simply not be relayed, let alone confirmed. You could of course manually connect to a specific node which has increased its mempool limits, but then your transaction will just sit in the mempool of that one node and not go anywhere, which is next to useless for you.