Is an extraction of the mnemonic string possible if wallet and corresponding passphrase are in possesion? My problem is that I have access to my wallet and I am able to access it with Electrum because I have the passphrase but I somehow misplaced the seed.

Is it possible to decrypt/extract and obtain the wallets mnemonic seed string with said access?

Any time donated towards this issue will be very appreciated.
From your Electrum toolbar click on "Wallet" and then select "Seed". A new window will open asking you to enter your password, do that correctly and you will see your seed words if you wallet has any (meaning if it is a standard HD wallet not an imported one).