Sorry to appear unhelpful, but I don't understand what the question is.
Is it
about meaning, grammar or what?
“I'll be back by Monday” will usually mean the same as “I'll have
returned by Monday”. The difference is in the functional sentence
perspective, in other words what main point is the sentence trying to
convey?
- I'll be back by Monday answers the question “when will you
be back?”
- I'll have returned by Monday answers the question “will you be
here by/on Monday?”
As you can see, the functional perspective of these two questions (and
therefore of the two answers) is different.
Trying to analyse individual sentences out of context is a nightmare.
Context is hugely important in understanding how English is used. Because we
have so many tenses, we need to have a clear idea of the time relations
in the context. And we also need to understand the functional
sentence perspective in the context. And with every noun, we
need to consider whether it needs an article or not, and if so, which article?
In order to decide that, we need to know if the noun is occurring as a “first
mention” or if we know about it already in the context.
[ After you have returned, after you are back are much less common
than when you have returned, when you are back – see the Ngram. ]