Answer to question 1) Petko uses one week to determine top performers so a 14 day free trial would be sufficient. I prefer to demo test for one month to determine top performers. In this case you can use two 14 day free trials.
Answer to question 2) Same answer as above.
Answer to question 3) When the EAs are created they are tested for various spreads. However, the historical data between various brokers and prop firms will be different. Commission variation will result in profit variations. The performance of an EA will be different when used with various brokers and prop firms. My recommendation is to test the EA on the same broker or prop firm you plan to use for live trading.
Alan,