Sandpaper! Do what you can to get those last bits smoothed down at least so the edges feather into the bare metal, otherwise it's just a future flake waiting to happen (and it will show and look funny, too). We replaced our wrought iron rail, but before that I painted it with an oil-based paint and it held for 9 years (even with Christmas decos ;) Just my .02, but I don't care for super high gloss (bounces a lot of light). For easy wiping, how about semi-gloss or even satin (depends on the brand paint you use).

The local rental yard should have the coring drill and core bits to easily remove the old section, and bore a new hole for a new one. The surface anchors are a very weak method for attaching railings. You have at least a 30 inch steel lever to rip the anchors out of the concrete with.

So there were a lot of questions but let me try to answer -If you keep the old hardwood it will be a real challenge to get the stains to match prefinished wood even if you redo the stain on the old floors. So depending on how much there is/costs, either replace it all with new, or do site finished for the new flooring so you can restain and match -IMO if the species of the wood is the same it's okay to have different widths. If they're all the same color you probably won't even notice where the size changes -Use the same species throughout the home so yes oak -if your home does not get good natural light dark wood is not a smart move. Not only does dark wood show EVERYTHING (that is not an exaggeration) but it will also make your home feel darker and heavier. A medium stain would be better.

I would ask the folks at Sherwin Williams about the paint to use and how to prep...they are typically very knowledgeable about their products.