What do I need to prepare for a German spouse visa?

Actually, your LG unit can do it for you. Otherwise, just apply for a tourist visa directly. Write a detailed itinerary and book hotels and air tickets.

As far as visas are concerned, it is no problem to go late and come back early, as long as it is within the time limit. Just ask the airline if it can change its visa and increase the related expenses.

The problem is not big. Take it easy.