Yes, you can make your own primer, but it will not be as effective as a standard primer. You can make primer by adding glue to the paint color of your choice; add 25% glue, 75% paint, and you are ready to go.
Some can be, yes. Most molds cause allergies that are uncomfortable but not life-threatening. However, all can be dangerous for people with asthma, compromised immune systems, the young or the aged....
Yes. But you must ensure adequate ventilation around the wood, reduce moisture ingress, and preferably protect the surface of the wood. Regular dusting, vacuuming, and wiping down also help.