Off-chain transactons, some questons

Hi I have some questions about grin usage as a currency.

Is it possible to make off-chain transactions, just by not posting the finalized transaction to the node network, but giving the finalized transaction to the receiver who can spend it again off-chain or on-chain?

My suggestions:
If that is possible, anyone in the off-chain-chain could do a double-spend. But that would be just be a problem, similar to other classic payment systems. And the classic solution, with a classic contract could be used for example.
Would it be possible to have side-chains or side-tangles?

You can do off-chain txs in payment channels.

Thank you for you answer, tromp.

Obviously I’m not familiar with grin protocol.
My Idea was grin could have an advantage when I compare it with BTC.
I imagine a situation where On-chain transactions are expansive because of very high demand and limited block size. In BTC, as I understood it would be possible to make transactions, even if they are not included in the blockchain directly. So they would be primary off-chain. It is possible to spend those transactions again and build transaction-chains. After some Month or Years they have chance to get included in the Blockchain because, someone in the transaction-chain is willing to pay higher fees, or there is a period with lower block-space-demand again. But for BTC, no pruning is possible, so the transaction-chain can not get smaller.

I thought, that with grin something similar would be possible. But because of pruning, the transaction-chain in grin, could shrink. So even if the transaction-chain gets more and more transactions, the size per fee could shrink. So there would be a 3rd way how it could find a way into the blockchain.