Translation:He said he will follow me to the end.
I think this is another case of the difficulty of translating a sentence without context. This is how Duolingo works, and despite it making it tricky at times, it also makes us learners think extra about linguistic constructions and how they vary between languages, which is, of course, beneficial to learning.
I can usually think of a context where the given sentence could work, even if it is not the first scenario that pops into my head. In this case, I could imagine say, a couple of running partners, where one usually runs the full trail, but the other usually takes a shortcut and runs a shorter trail. But, this coming weekend, the second person has some extra spare time so this time "he said he will follow me to the end" - of the trail, as it were.
"would" is not used exclusively in conditional events in English, but also in future events in reported speech when the reporting action is in the past (this case) and in past habits.
In this specific sentence, if "he" already started following the subject, "would" should be used. If not, "will" should be used. Reference: https://dictionary.cambridge.org/us/grammar/british-grammar/reported-speech-indirect-speech
As an English teacher I agree, except that I would say that "will" may be used, not that it must be used when the event is still in the future. In your link it says backshifting is not necessary if the action is still in the future at the time the reported speech is made. It doesn't say that that maintaining the future tense is required. In fact, English speakers often use "would" in both of the scenarios you mention. Duo should accept both "will" and "would".
Yes. The correct translation to English depends on when the sentence is said.
- before he starts following me: He said he will follow me to the end.
- after he starts following me: He said he would follow me to the end.
"to tell" is usually used with an indirect object (the person whom the speaker is talking to), except in fixed expressions such as "to tell a joke / a lie/ the truth / a story". In the first clause of this sentence ("ha detto"), there is no indirect object.