Yes, it is repairable.
No, there isn't a manual or an online exploded view (Shimano's exploded view only covers partial disassembly) to help you. (and the amateur one that does exist, misses out a key and critical section - ETA - don't trust everything you read on the internet)
So, you will need to gain an understanding of how it works, by disassembly and investigative experimentation / assessment.
You will need to make some tools (spring retainers etc) to accomplish reassembly.
DNAHIKT.
I had this a while ago, when I was time-rich, but cash-poor, and needed to repair a Sora LH lever.
It is do-able, but I wouldn't jump at the opportunity to do so again.
Once you've established that there isn't any rogue inner cable jamming up the works (as per Flatus's advice) I'd look at buying a replacement.
80, or so hours, and multiple, multiple re-builds during that time.