All you can do is experiment, really. Passing boiling water over the onions will make them sweeter -- the heat will bring out some of the sugars and tone down the acidity, and it will soften them a little. If you want your onions are crunchy as possible, I wouldn't. If you prefer them slightly softer, like silverskins, then I would.
Not too sure about reusing pickling vinegar. Part of me would worry about contaminants that had got in there while the original pickles were being eaten. Pickles don't keep forever, and even vinegar has a shelf life. If I were making pickles to give to someone else I'd use fresh vinegar and make up a pickling mix. If it were just for me, I might experiment.
Sam