The amount of lost shares in p2pool are normal. And every p2pool miner has the same. So its fair among the p2pool users.
Theory: But that means that a certain amount of hashpower is lost compared to solo mining. The shares maybe were good but they are lost and so is the amount of time invested in them.
Solution?: Am i right that this is no disadvantage to solo mining because the real bitcoin block that is found in p2pool is directly put into the network via solomining? So the lost shares does not affect in the end because for the real found block those lost shares dont exist. Correct? Then p2pool would be even better than normal pools because they have to receive the found share first and from there propagate it.
Sorry for the many questions...

no problem, they are good questions - some are answered in other places but it can be hard to find.
Yes, p2pool's internal share chain has lots of stales. But the p2pool network DOES NOT produce more bitcoin chain orphans (and thus wasted work) than solo mining.
Work is not lost using p2pool, those stale p2pool shares can still solve a block and still be submitted to the bitcoin network for reward. The time and work invested in them is not lost. Those shares just don't show up in the p2pool sharechain, but will show up in the bitcoin blockchain. The rewards for those blocks are paid out normally to p2pool users and are not wasted.
Theoretically, a 100% efficient p2pool miner that has ~5-10% stale/DOA rate will have the exact same expected value as a solominer with the < 0.5% orphan rate.