Horizontal pulls work different muscles.
The simplest thing to do working up to pull ups is eccentrics. Jump up into the flexed position and use your muscles to work against gravity and control the descent. You can almost certainly pull that off and it doesn't require anything you wouldn't already have if you're trying pull ups.
You get diminishing returns the more weight you lose, that's just how it is. The question is where are you comfortable maintaining your weight. Being fit is a lifestyle, and your body will be in the shape that your diet and exercise gives it. So you have to figure out how stringent you think it's worth it to be in the long run vs how it makes you look/feel. If you just cut down real low and don't maintain a regimen to suit that body, you will bounce back to whatever your regimen will