Hot or Cold . I don't know if you'd get enough hot water using the cold supply or not but I'd try that first . If it didn't work I'd switch to the hot supply . Cutting the small faucet supply pipes above the shutoff valves , using flexible copper tubing & compression fittings it wouldn't take much to hook it up or switch supplys if necessary . You'd likely need reducers to connect the tubing to the inlet & outlet of the tank .
Phew! I'd rather boil up a pot o water on the stove and bring it into her than install that POS point of use electric tankless.
What part of 0.5 GPM don't you understand?