The least controversial part is what you are investing your time in these episodes:
>I think in earlier seasons he was written more as a sidekick or even Pet without a clear family bound fully in mind. well, it wouldn´t surprise me at all, considering that Spike was seen as a less relevant character than the Mane 6 and he didn´t get a more serious treatment until future episodes. Correct me if I am wrong here but, wasn´t Spike shoehorned in the cast because of Hasbro´s orders to the show staff? I am asking this because Faust could have seen herself a little bit under the obligation to make something with him but didn´t aim all that high for him. One of his biggest achievements happens when the Crystal Empire events occur but the staff was changing plans constantly around that era and Faust was no longer involved in the show´s direction. I also remember (this is outside the show by the way) that Tara Strong posted a tweet claiming that Spike was her pet so...
indeed, you are aiming correctly with this speculation.
> TS was shown to be his caretaker and those moments caused plenty to argue back then there was such a type of relation. Even here TS was acting as such. Though I wonder if there is any hints with a clearer subtext of motherhood or sisterhood implied. I believe that the show wouldn´t go beyond those subtleties. I mean, Sparity would be a more obvious trait in the first seasons than that but if we were to look at the root of being shown as a caretaker, the main source for establishing that theory relies on the backstory when Twilight hatched the egg with her magic for passing the test (Cutie Mark Chronicles). I am not saying that those implied moments don´t exist but I think that the show was sustaining itself in an unclear path around that era.
My theory is basically this one: Spike was slightly detached from the Mane 6 in terms of writing in general and seems kind of like an artificial added member (familiar yet slightly alien at the same time) until they start to appreciate him and what he can actually offer (both in terms of writing and for the mane 6). If you see the rise of relevance for male characters, you see that him and Big Mac are treated with a deeper take as the show goes on.
Even if you leave aside the perspective about male characters in terms of writing, Celestia suffers this same problem as well. Like you say with Spike as a friend or as family member, they don´t show you much about Celestia either. Was she supposed t be the most mysterious princess that knows everything but also trolls at the same time? Was she supposed to be a mentor for Twilight? Besides her presentations and a few lines dropped back and forth, the show was lying in a fine line where something was lacking beneath its surface level. She was kind of there and stayed like an alien yet familiar for the mane 6.