is it compulsory to use Keychain to login to steem-engine before the claim?
You can use steemconnect to log into steem-engine as well, so Keychain is not necessary, just recommended.
is it also possible to reward people's post with palcoin by voting them from other Steem front ends if one hold a PALcoin staked?
Yes it is, but only if they used the palnet tag.