Where to start?

First, stains only stain bare untreated wood. They do not work on already stained or finished wood. There are 'stains' called gel stains. Those are paint, not stain. They are, however, your least expensive option for darkening the balusters.

You cannot stain---gel or otherwise---lighter.

The other staining gotcha is that different types of woods stain different shades when the same stain is applied. Meaning nothing will be the same color.

I've never seen bull nosed plywood treads. Treads are usually at least 1" thick, and more often 1&1/8" thick if made of hardwood or 1&1/2" thick if made of construction lumber. Construction lumber treads are designed to be covered by carpet. There is a good chance someone will get splinters from construction lumber threads at some point since the wood is way too soft for staircase wear.

You could sand all the finish and most of the stain off the balusters and spindles---that would take more money in sandpaper than it would cost to buy new spindles.

End advice is to get new materials.