how long can the free-trial last?
There is no time limit. You can keep using the free-trial as long as you do not go over the limit (1 million call & 1 million push per month, 1GB storage).
Is there any limitation to API-call per second?
Please try to keep the API calls below 20 req/sec. If you think you need more calls, please consult firstname.lastname@example.org
For the Social Referral feature, is it included in Unity SDK as well?
No, the social referral feature is not available in Unity SDK.