Answer: The easy question first, marigolds need full direct sun all day long to thrive. In my experience they just do not get enough light indoors on a windowsill, not even south or west facing windows.
If you want a flowering houseplant you might try African violets as they are well suited to growing and blooming indoors. They need a bright location, but you may have to use a sheer curtain to filter the light coming in a west window.
Damping off usually happens when seedlings are overcrowded and/or overwatered and/or grown in too little light. They should be spaced far enough apart that they have room to grow without touching. The soil should be slightly damp like a wrung out sponge -- but not sopping wet. Also, starting with clean tools and eqipment and using a good quality soilless potting mix can help avoid the problem as well. If you folow general seed starting directions carefully you may never see damping off.
If damping off occurs the seedlings are usually lost. Sometimes you can stop the problem from spreading to nearby seedlings in the same pot by spritzing them with chamomile tea (brew the tea as if you were going to drink it and let it cool before using) but usually the plants are so stressed by then that they are not worth saving.
Here are some links to seed starting information. You may need to cut and paste the complete url into your browser to make it work correctly.
I hope this helps. Good luck with your seedlings!
Q&A Library Searching Tips