You might be able to use something like MQTT 2.5 M1+ How to implement the equivalent to MQTT1 REGEX filters.
Use a REGEX to filter(s) out those messages that are not appropriate for the given Channel and only when they pass the REGEX filter(s) will it get passed to the JS or at that point JSONPATH transform may be sufficient.
Yes you can. This is exactly what rossko57 is describing in his second paragraph. What we don’t know is if MQTT is set up to support that though.
See my link above. You must be running MQTT 2.5 M1 or later though.