If you have a bit of isopopyl alcohol you can pop off the metal nib and drip a bit in there to revive the dried out ink. There should be a little notch in the plastic where it meets the metal nib. You can take a needle or thumbtack and wiggle it in that hole. It should come off pretty easily. You can look up videos on youtube for how to refill microns. They'll show you how to do it.

I wouldn't say it is necessary to redo the ones you've done already. You'll revisit the exercises in the future for warmups anyway.

Tips:

Keep the pen at a 90 degree angle to the paper. Don't press hard. I've found that the superimposed lines exercise does a real number on the felt regardless of hand-pressure. Something about the repeated, fast, friction against the ink already on the paper causes it to destroy the tips particularly fast in my experience. ymmv I generally don't revisit that exercise too often. Instead I just do parallel lines for warmups(or use a different pen).

As an alternative I've also used Pilot Precise V5's and thought it was pretty comparable line-quality wise. If you are in the US you should be able to find some at your local big-box store. It works out to be about $1.50/ea The ink isn't archival quality so it will fade to blue after a while. Also be aware that using a rollerball pen can leave trenches in the paper that can throw you off. So again, don't press hard.

I hope that helps! Best of luck and stick with it! :)