@1234muse
Him being a self-insert as you've so idiotically proclaimed and him getting involved in situations are two different things, first of all.
I can't say much about the powers beyond Imagine Breaker because even us novel readers don't know all that much, so unfortunately I can't really discuss that aspect.
About plor armour - every main character has some level of plot armour, no matter what series they are in. Hell, Accelerator and Hamazura have plenty of that themselves. I'd even say Mikoto has plenty of plot armour herself (especially in Railgun, as the arcs here are light years behind the light novels and as we know this all takes place in the same universe, literally nothing bad can happen to any of the main 4 because they've been seen way later down the road in the novels.) I should also add that doing something terrible to Mikoto in story (like killing her off) is just going to result in her rabid fanbase losing their shit over it and honestly wouldn't even surprise me if people were to send death threats to Kamachi over it lmao. You can complain about plot armour all you want, it's always going to be there. Also, there's more often than not a reason why we are following a certain character in a story (like Touma). Killing the main character would be effectively putting an end to the story as a whole.
Touma being a crutch in the story is honestly no surprise, especially given Mikoto's heavy involvement with him throughout the Index novels (she gets even more involved even when she's honestly not even relevant to the story at all.) Mikoto's always wanted the strength to be able to protect those she holds dear and this becomes more apparent in the novels, when she's going up against opponents she has no chance against. She hates the feeling of being powerless while the people she cares about are being injured in battle. The most obvious example being Touma's fight with Accelerator.