transaction fees – Is it even possible to spend a stash of 1-sat UTXOs?

They are economically unspendable. If you only had the described 1-sat UTXOs, you would be unable to build a transaction that meets the minimum feerate.

Each UTXO requires some blockspace to be spent. If this blockspace costs more in fees at a given feerate than the UTXO is worth, the UTXO negatively contributes to the fees of a transaction—it cannot carry its own weight, other inputs would need to add more fees to cover the deficit. I describe such UTXOs as having a negative effective value (at the given feerate).

As in your example, a UTXO might have a negative effective value even at the minimum feerate. I call such UTXOs economically unspendable. They can be spent, though only if the user deliberately pays extra for removing them from the UTXO set.

^{Note that the above terms differ from dust: the dust limit refers to the fixed amount of fees an input and output of a specific output type would cost at the discard feerate (3 ṩ/vB). The limit that makes an output economically unspendable is based on the cost of the input at the minimum feerate, and having a negative effective value is a property depending on the context of a given feerate.}