1) I don't recommend DIY binders. Commercially produced binders are designed and tested, and thus safer. If you have cash or a debit card, buy a prepaid Visa giftcard and use that to make your purchase. That is what I would suggest.
2) Binding breaks down breast tissue, so over time your chest would become less firm and more malleable. It has no effect on top surgery. I wore a binder for 8 years and my surgery turned out great. Developing your pecs will help a future surgeon know where to cut things, so that is what I would focus on doing in the mean time. Long term binding can affect your posture and breathing. If you bind unsafely, you are at a greater risk to hurting yourself.
3) Weight loss could potentially shrink your chest. Weight training won't do anything but build up your pecs which wouldn't be visible until after surgery. You could try wearing two sports bras and layering your clothes over top.