There seems to be an unhealthy level of suspicion regarding ElectronCash... providing you have downloaded the official ElectronCash app from the official page:
https://electroncash.org/#download you should be fine to use it. The source code was forked from Electrum and is open source. The commits listed on the github don't add anything nefarious into the code base.
And if you're concerned about running pre-built binaries, you can easily run it from source and/or compile it yourself...
Having said that, it is still a good idea to move your BTC prior to attempting to get your BCH, simply because getting your BCH will require you to either enter your seed or export/import your private keys... Both of these procedures could potentially compromise your wallet and coins. So, if your BTC is already moved, you know it is safe and only your "forkcoin" is potentially at risk.
Sound risky to me, is there any other wallet which is 100% safe for BCH and support Electrum seed ?
The only thing that supports Electrum Seed is Electrum and forks of Electrum... like ElectronCash...
No-one is going to be able to give you a "100% safe" guarantee for any bitcoin wallet. If ElectronCash was unsafe, we'd know about it by now... the initial rush after the fork and the recent big pump and dump have seen so much BCH action that if ElectronCash was doing anything "bad", this forum would be full of threads telling us.
EVERY time someone has had coins stolen is because they exported private keys on a compromised PC or downloaded a fake version of the wallet. I've not seen one substantiated claim that ElectronCash stole someone's seed/privatekeys or coins in the 3 months that it has been operational.
If you're that concerned about it, install a VM like Oracle Virtual Box... and create an online/offline system... so the "offline" ElectronCash system restored from seed is on the VM with networking disabled.
If I export all of my BTC address' private keys from Electrum then add them to blockchain.info will I be able to use all the BCH I received after the hard fork ?
Assuming b.info supports BCH then yes... you should be able to see it. I would say that exporting your private keys and then entering them into a website is more risky than using ElectronCash.