Zero. It is exactly as PC has said, they don't reject any trades. And no orders are 'lost', nor markets closed or 'telephone only'. PC seems to me to be an honest and open character, why don't you accept his repeated assurances on this?
Just watch the last minute of a session, they seem to close markets between 20 and 60 seconds before 9pm, (or 9.15pm, as applicable). Although it is not apparent until one opens a fresh deal ticket or attempts to close a trade.
As for slippage/price improvement, I would expect it on market orders where the market moves in the fraction of a second between my press of the button and cmc receiving my order. It seems to work in my favour as often as against me. I would very rarely expect it on stops. As you say, if the stop is held with the market maker on their system, then it's rarely that slippage is justifiable, other than around major news event type spikes.
You seem to have a City account, are there regulatory issues regarding trading from Sweden, that City have overcome and cmc have yet to?