How should I clean old plates?

Wash them by hand in warm soapy water using a soft cloth. Modern dishwashers use harsh detergents and high heat that strip the delicate overglaze enamels and gilding. Avoid leaving them submerged for long periods. If the base has an unglazed foot rim, water can seep into the ceramic body and cause staining.

I have heard that lead glazes on early ceramics can be dangerous. Can I still use these for serving food at a dinner party?

Most antique porcelain plates are safe for occasional use. The high firing temperatures used for hard-paste porcelain seal the glaze, preventing dangerous leaching into your food. Acidic foods can sometimes react with soft-paste glazes. If you plan to serve citrus or vinegar-based dishes, it is safer to use modern dinnerware and display the older pieces.

Can they go in the microwave?

Never put them in a microwave. The metallic gilding around the rims will spark. This causes permanent damage to both the appliance and your antique porcelain plate.

Some of the dishes I bought have brown crazing lines across the white background. Is there a way to bleach these marks out safely?

Crazing happens when the glaze shrinks faster than the clay body over time. The brown lines are organic matter that has seeped into those microscopic cracks. Bleach will weaken the structure of the plate. It is better to accept the staining as part of the history rather than risk destroying a brittle ceramic.