I will answer your questions in the order you posted. Maintenance excess represents the difference between your Market value and the maintenance requirement (which is usually 40% of your Market value, depending on the volatility of stocks you have bought). Your buying power is twice the maintenance excess. You do not have to have cash to buy on margin, but you can use only up to your buying power. Any time your Market Value drops below 2.5 times your Maintenance requirement, you will receive a margin call on the difference. So for your example, if you buy $10000 worth of stocks, your numbers will change as follows:
Cash: -7246.00 (that is 2754-10000)
Market Value: 23336.00
Maint. Excess: 4712.50 (that is 9712-10000/2)
Buying power: 9425.00
To maintain 23336 on margin your net account value should be no less than 23336/2.5=9334.40, and according to your numbers, yours will be 23336-7246=16090. Now, if your market value drops to 12076 (from 23336), you will receive a margin call, for your net account value will be 12076-7246=4830, and since 4830*2.5=12075, it will not be enough to maintain 12076 on margin. Your interest on margin will be (your negative cash balance)*0.072/360 per day, in your example it is 7246*0.072/360=$1.45 per day. I hope this helps.