A lot of people think that there is no substitute for pullups/chins and I agree. But that doesn't mean the same applies to rows and it also doesn't mean there won't be other exercises that will be useful down the road. I do lots of different rows and things like that.
Seems to me like everyone is basically being put in a cage by relying on one or two people's opinions about the "one best exercise" they can do in a certain category. When someone tells you that their choice is the only effective choice it's time for
The assisted pullup thing sort of creates a hybrid movement. The best idea I've ever seen for assisted pullups is to use a thera-band or some elastic lifting band along those lines. You put them around the bars and you basically stand in them so that they assist you. Would be tricky to get into....
One thing that is helpful is to have a pullup bar that is not so high that you can't stand and grasp it overhead. That way after your pullup set you can add in some negatives by doing jump pullups. Usually after you fail on the postitive you can still get some negs in and that will help make you stronger. Keep in mind that the risk for injury in negs is high so only do what you can do under strict control and don't push it. But you wouldn't be doing "heavy negatives" so it's not to bad.
Another thing is if you wan't to have more success at pulls then drop the curls for now. Here is what I've discovered. Pullups make you better at curling heavy weights but curling interferes with pullups a lot of the time. You need you biceps and if you wan't to prioritize pullups (and rows to some extent) then take out the unnecessary stuff.
At the end of a pullup session try switching to chinup grip to get some more in and further strengthen the biceps.
And last but not least. Pullups are much harder after overhead press than after bench. Other people have told me this and I, being a pullup junky, have always found this to be the case.
Try different ways of dong the pullups. For instance instead of doing 5 in a row only to fail at 3 or 4 after the second set try to get more reps in by doing double or something like that with rest in between. Don't add weight right now although some people might advise that. Given the amount that you can do I would be afraid that added weight might bring on some elbow tendinitus or something. You need to get the joints and tissue ready for more weight so I like more reps first.
If barbell rows are giving you fits then pick something else. There are lots of good choices. I do one arm DB rows all the time. I also like to do DB rows on a bench. Cable rows. All sorts of different angles on that (for you just stick to one "neutral" angle right now). Barbell rows have gotten to the point where I simply can't do them without my lower back being irritated and it's not worth worrying about. I do so many deadlfit variations and posterior chain things I don't feel I need to worry over not doing a barbell row.