Asriel’s hesitation is rooted in fear. They say young kids feel "invincible" which is why they'll take on risky things. That's why young Asriel reassures himself that everything will be fine and takes the soul. But older Asriel actually recognizes his fear and allows himself to be stopped by the crippling anxiety over the whole situation.
Yes, this request has already been made. Since the events of how they fall ill up until their death would be relatively the same as in canon (asides from occurring years later), we opted to leave this to the imagination.
Chara isn’t in the void. They are dead. This dark space may look familiar. The difference is in how the soul is represented.
Actually, Chara is wearing the shirt from the game in the interlude, Rewrite. Notice how their green and yellow shirt has long sleeves, just like Chara’s sprite.
