Dependence and dependency can both be used in the state/condition of being dependent sense. By definition, all words referencing such "states" are abstract nouns, so I don't see any justification for OP's abstract/concrete distinction in that sense. As you can see from this NGram, dependency has gained ground in recent decades, but both are in common use.
The main usage difference is that dependency can be used in a second sense as a "concrete" noun to mean a person or thing which depends on something/someone else. But note that in the programming context it's not uncommon to see it used to mean a software resource upon which some piece of software depends (i.e. - reversing the need/provide relationship).
In principle, dependence could also be used with that second sense - but as OED points out, all such usages are now either obsolete or archaic.
TL;DR: If you want the easy way out (which looks like the way majority usage is going anyway), you can probably get away with using dependency all the time.
But I must be honest - as a native speaker I'd probably tend to refer to his drug dependency, but his dependence on drugs (maybe because I see one as a problem he has, and the other as something he's doing, I don't know).
In the most exact sense of the word, "feel bad for" implies empathy, whereas "feel sorry for" implies sympathy.
In everyday language (in my opinion at least), both of these expressions can be interchangeable, but not always, so it's one of those situations where context is key.
"I feel bad for Bob" and "I feel sorry for Bob" can both mean that you sympathise with Bob's situation, for example if Bob had just lost his job. Unless you are very close to Bob, it's doubtful that his situation physically makes you feel ill, but a native speaker will interpret it as being sympathetic to his loss of employment.
However, "I feel sorry for Bob" can also be interpreted as having pity for Bob, depending on the context. For example, if Bob was in a situation where he was being naive, lets say in a relationship where his friends felt he was being taken advantage of, then one might say that they feel sorry for Bob. Depending on the tone used, "I feel sorry for Bob" may also imply an element "well we did warn him, but he didn't listen", particularly if it's said after some predictable calamity occurred, for example if Bob's new girlfriend just ran off with his best friend.
Best Answer
They mean the same in those two examples, and both are grammatical, but I dislike as per. To me it sounds stilted and I can think of no occasions on which it is necessary to use it. That, however, is a personal view with which others may disagree.