If you use a stripper to remove ALL the finish---and then sand,------stain will not penetrate through finish---you can apply more stain. Matching the damaged areas may be a problem---requiring different concentrates of stain.

The best tool to use would be a random orbit sander(ROS). Do NOT use a belt sander!!!!!! After the finish is stripped off(you could sand it off, but that will take a LOT of sandpaper---the finish clogs sandpaper a LOT!)---sand with 150 grit paper(keeping the sander pad flat) until the surface is smooth.

Apply stain with a rag---or a paint brush and wipe off with a rag. That lets you control the amount of color and will help blend the color in the damaged areas.

Let the stain dry for 24 hours, then wipe thoroughly with paint thinner on a rag---gets all the excess stain/sawdust off.

Apply an oil based varnish(preferrably not a poly varnish(read the can carefully). That finish nis much easier to spot repair than polyurethane.

Start with a heavy duty chemical paste stripper like BIX paint and epoxy remover. Brush it on and let it sit until it causes the original finish to wrinkle. Scrape off the goop with a plastic tool. Wipe the surface clean with a solvent dampened rag.

Not all orbital sanders are equal. Most are junk. For under $100, the Bosch is the best I've tried. The junk ones don't spin continuously when in contact with the work surface. They just sort of vibrate in a mini-orbital manner, and the end result is a pattern of mini-swirl marks all over the work piece. A good orbital sander continues to spin the entire pad in a circular motion in addition to the random orbital micro-action. When you use an orbital sander, hand control is key. Your hand's job is to keep the sander pad flat against the work piece, and slowly move the machine across the surface back and forth. I always see people rolling the sander up on the edge of the pad, rubbing back and forth vigorously like they are using a sanding block. This will ruin the work piece by leaving bellies, dips, and swirls everywhere. This bad habit is learned from using junk orbital sanders. Get a good one from the get go and then learn how to use it the right way- let the machine do the work.

I could in all likelihood remove all of the varnish on that table top in under five minutes with a cabinet scraper, and be applying the new finish in another 15 minutes, after a light hand sanding.
Casey