1. How can maintain the largest army in Europe(for now), without going bankrupt?
Get rid of peasants, disloyal units, units lead by drunkards, etc. and get you some trade.

Also, Scottish highlanders are amazingly cheap to both build and maintain.

2. Who can I trade with, and also how do I trade over sea?
Trade with everybody you're not at war with. You will need an unbroken chain of ships, ports and trade centers. These are quite expensive to build, by the way.

3. Where to expand next, north against the French who have a couple of provinces left or the Germans who are at war with the French? Or South against the Egyptians, or the Byzantines, or the Italians (Navy and all).
I'd conquer Egypt, which will help boost your income.

4. Should I keep low in the eastern provinces for now until the Golden Horde come and duke it out with the Northern People/Byzantines/Turks and then when they're gone move in?
Don't worry about the horde. They are about to do you the very great favor of weakening many of your remaining enemies in the east. This is another reason for driving on Egypt. Get yourself in position to exploit the chaos that comes when the horde hits.

5. How to maintain a steady/increasing flow of cash?
By trade.

6. What to do next?
Build that trade network.