Not having traded CFDs before I'm probably completely wrong, but here goes:
1) Trades can be kept open indefinitely, however financing charges will be incurred, therefore making them inefficient for long term trading (investing).
2) Not having traded, I can't comment on CMC or any of the others operationally, however a significant difference between them & the likes of GNI, is that with CMC you pay an increased spread, as opposed to commission (0.25% of every trade with GNI, I believe), although with GNI you will also pay the current market spread. You should be able to arrange a free trial of the various options available, therefore getting a 'feel' for the platform that will suit you best.
1. CFDs can be kept open as long as you wish. You will pay daily interest to D4F on any longs held overnight and D4F will pay you daily interest on any overnight shorts.
2. D4F claim to offer the market spread on UK stocks ie there is no increased spread. Generally this is true but may not quite always apply eg the spread on RBS on D4F is never less than 2p, but then the share price is currently £15 - $16. There is no commission to pay with D4F.