- why is it あった and not ある?
Presumably C (or somebody else) was describing something to A just a little bit before this excerpt. As such, the あった refers to what was just said -- "was there something you didn't understand (in whatever was just talked about previously)?" If the speaker had used ある instead, the statement would be a bit vaguer -- "is there something you don't understand (in general, not necessarily in connection with whatever was just talked about previously)?"
- is こと completely interchangeable with とこ? I see a lot of them in the novel like this another one 俺は壊れたとこがある (which also a complete mystery of a sentence)
No. とこ is a shortening or clipping of ところ.
- what is the meaning of 私 にわかる?
わかる, like できる, is often translated into English in the active voice: "I understand something, I can do something". However, the Japanese words more accurately describe a quality of the "something": "something is understandable to / by me, something is doable by me". This is why the "something" in both わかる and できる sentences takes the subject particle が instead of the object particle を. The agent (person or thing doing the action) in the English sentences is the subject, and the patient (the "something") is the object. However, in Japanese, the agent is the "something" that is the subject marked with が, and there is no patient or object. Instead, we either use the topic particle は for the person, or the instrumental ("by") particle に.
More broadly, わかる and できる are verbs of potential, which take this same grammatical construction in Japanese. See more about potentials in this question and its answers:
Why doesn't 分かる have a potential form?
- if 私 にわかる translated as "Want to know from me about it?" then why けど at the end which translates as but/however? (even if my translation of 私 にわかる it still doesn't make sence for me why it's translated like that)
See the previous point about the にわかる construction.
With that in mind, here's a breakdown word-by-word, then a reformulation of that to get a translation in proper English.
me by understandable place/aspect if teach/explain but →
If it's something I understand, I'll explain it [to you]...
Please comment if the above doesn't fully answer your questions.