I won't tell you anything other than that it's NOT Satella.

Having Rem on that poster is not good sign when it comes to following source material...

Completely agree on this I hope the studio doesn't change her status on S2 but who knows at this point.

The poster isn't always the best representation of what the actual anime is like. It's probably more about having of the better-known characters from the series be on the poster than them actually changing her status.

Try not being so negative off of a simple poster, it'd be more spoilerific if they removed her from everything.

They probably included Rem in the promotional material because it'd be crazy of them not to. She's easily the most popular character in this series and has the biggest fanbase. Not including her will definitely hurt their PR.

Besides, her absence would also indicate that something's up (aka an unintentional spoiler) and if they want to preserve the feelings of legitimate surprise in viewers, then they should continue to nonchalantly promote her as tho it's business as usual.

Also, the ending of the first season isn't too different from the LN/WN either. The events that are supposed to happen, can very much still happen during the first 1-3 episodes of the second season. I expect season 2 to pick up literally right where the first season left off.

While most on this subforum would know what happens in arc 3, according to where the anime left off, anime viewers are under the impression that everything's fine atm. The fact that there are some people in this thread who don't know who Echidna is, proves it. It's precisely because of them (anime viewers/manga readers) that the promotional material is presented the way it is.

tl;dr, I'm not too worried about it. The thing you're doubting, I very much expect it to happen within the first 3 episodes of the second season.