Okay with no experience with these tools, I would definitely not recommend attempting a cut into or soldering onto such small screws...
If it is necessary for you to remove the screws in order to complete your keyboard replacement, I would look into using some Screw Extracting Pliers ([product|IF145-127]) or Precision Screw Extractor Set ([product|IF145-118]) both run for about $20-$25 USD. One of those may be a better tool to use rather than a rotary tool or soldering iron.
You'll want to remove the entire screws which are striped and replace them with ones that are not in case you'll need to remove them again in the future and to better be able to install them again to finish your repair.
With the pliers or extraction tools, you should have the best grip on the head of the screws to remove them.