i just wanted to make sure, whether the electrum wallet will be backward compatible in near future ?
Nobody knows what is going to happen with Electrum wallet in future, but that is the beauty of open source projects, anyone can contribute or fork it if they ever stop with development.
e.g. bip39 or some other.
It's no mistake if you choose electrum or Bip39 standard.
Bip39 standard is supported by most wallets included Electrum, and you can always generate seed words totally offline with dices for example.
With Bip39 you are not limited only to Electrum, but you could also import them to most hardware wallets or you can use iancoleman's mnemonic code converter.
or should i store copy of electrum's source code along with my seed phrase, for future use ? you know for worst case scenarios ?
There is no need or benefits for storing electrum source code with your seed words.
so hypothetically, lets say i die in this 15 years timeframe and some family members find the "titanium seed storage", then it should be easier for them to recover the coins too.
Most issues I saw when people are trying to recover wallets with using of non-default derivation paths, so I would make sure to engrave that along with wallet used and seed words.
You need to provide good instructions for your family members and explain them how bitcoin works, better sooner than later.