“daughter-towns.” The Hebrew text is just “daughters,” referring to small close-by towns that are supported by a “mother” town, a large and normally well-fortified town (see commentary on Josh. 15:45).
“300 years.” Jephthah’s point should be well taken, because if the Ammonites did not try to take their supposed territory back from Israel in 300 years, maybe that was because it was never theirs in the first place.
“rescue them.” This is tongue-in-cheek. Jephthah is making the situation more obvious by using irony. The towns did not need rescuing because they were not Ammonite in the first place.