A lot of English translations render the words “ἐν ἐσχάταις ἡμέραις” as “in the last days” at James 5:3:"ως πυρ" belongs to the previous sentence.
The original Greek doesn't say "For the last days" but "in the last days" (εν εσχάταις ημέραις). This is not necessarilly eschatological, but it may refer to everyone's personal judment day, i.e. after his death.
There is no definite article in the following passages also:Yes. Actually the gr. original doesn't say "in THE last days" but "in last days". I 'm not an expert but I would understand it as an idiomatic expression, meaning "in days (that may be) last", or "in days that are as if they were last". Most probably doesn't refer to a particular fixed "last day". In a sense, anybody may die soon, which practically means the end of his/her chances to accept Jesus, or repend etc.