Will someone have access to your wallet without your device but the person manage to see your private keys and copy them without your knowledge. Will the person be able to withdraw your money or change password and block you from having access to your wallet?
Absolutely Yes, Have you not heard --> Your Key, Your funds? Even if you lost the key or someone stole it from your it's no more your key and anyone with the seed phrase can open your wallet and can change the password but AFAIK he/she can't change the seed phrase because it's fixed. So, you can again use the same seed phrase to recover your account but what use it will be then, as the unauthorized person must have already taken all of your money and flown away?
Keep in mind
Key is everything without it you are no more owner. I have seen many cases in which people forget to store their keys in safe places, even forget to save them in any place and when they need them, and can't find them. Then they realize, that they have lost their funds. As, according to stats and history, Many Dormant BTC addresses (the ones which are not active for a long period of time) is a live proof of Lost Wallets, which means people have lost their wallets/keys to their wallets and now they can't access them. Following are some Dormant Addresses that aren't active (maybe some have become active).
Most of the analysist says, 20% of the supply of BTC in circulation is wasted because of lost of keys and they have become dormant BTC addresses. But this is just estimation, because no solid stats are provided so, DYOR too.