You are spot on with respect to omnipotence, it is logically impossible. On the question of omniscience I think your argument could use some improvement.
The issue I see is the definition of what it means to "know" something. Knowledge implies a degree of certainty as to the verity (truth or reality) of the subject. Thus you can know that 2+2=4, and that 2+2≠5, but you can not know that 2+2=5 because that is not a true statement. Thus, it is impossible to know things which are impossible because those are false statements. On that count an omniscient being would be safe.
However, it is possible to disprove omniscience on purely empirical grounds. In physics there is something called the Heisenberg uncertainty principle which states that we can not know the exact position & momentum (velocity) of a particle beyond a certain amount. The more exactly we know one quality the less exact we know the other, & vice versa. It is important to note that this is not a technological failing; it is impossible to know the exact position & velocity of a particle EVEN IN THEORY. The result of this is that even though nature follows deterministic laws, there is real, genuine randomness in nature, and thus, it is *impossible* to know the future with absolute certainty. Thus, at least in our universe (and god is supposedly present here according to most theologies), omniscience is impossible.
Here's another problem with omniscience, as it relates to the problem of evil. The problem of evil is thus: if god is omnibenevolent (all-good) and omnipotent (all-powerful), then why is there evil? God should not tolerate any evil if he is all-good, and can remedy or prevent it since he's also all-powerful. So the existence of evil means that he is either not omnibenevolent, or not omnipotent, or neither. The typical xtian response is that god gave us free will so that we could choose our own paths and our own ultimate fate after death. There's at least two problems with that though. First, that means that god values our free will, including the ability for us to choose to do evil, more than he values being good. Thus, god can not be omnibenevolent. Second, if god was omniscient (an impossibility as demonstrated earlier, but let's say for the sake of argument), then he already knows everything we're going to do and choose. This means that since our decisions & actions are known in advance, we do not and can not have free will. Having free will means that when you make a choice there is more than one possible outcome. But if god already knows in advance what your "choice" will be, then there was only one possible outcome to begin with and your "choice" was at best an illusion.
So let's see...we've demolished omnipotence, omniscience, and omnibenevolence. Seems like there really isn't anything left we can call a god, at least not in the Abrahamic (xtian/jewish/muslim) sense of the word.