Elizabeth said:
Does this convention of not using いっている about a third party still apply
though if the person clearly hasn't left yet or there is context on time in the sentence ? :? 今月下旬から、彼は二週間に東京に行っています。
When I hear 今月下旬から、彼は二週間に東京に行っています。 without any context, my understanding will be
1. he's already gone to Tokyo and the duration of his stay is 2 weeks
(Could be less than two weeks from now, as he's already left.)
2. It is already in the latter part of the month.
The reason I feel that he's already left is it sounds 「彼は（すでに）東京に行って（しまって = completion、今は東京に）います」.
To me it could mean what is going to happen in the future (i.e. he hasn't left yet) only in a specific situation - talking about him with another person when he is in sight, or it is clear for us that he hasn't left yet.
Then this sentence is about what status he will be in the future.
(But still, we'll be talking about what status he will be when the action of going is completed.... We see the situation from the future point of timeline, by the time the action of 行く will become the past.
)
But basically Verb A + ている means "being in a status as the result of doing A (= A has been already completed)" as far as A is a verb describing an action or movement which is just one go, something you cannot keep doing. Other posters mentioned the verb groups, I think they're talking about this point.