If you would like to request a Blockchain API key, then you can do via this form: https://blockchain.info/api/api_create_code. Just let me know when it is submitted, and I will approve it for you.
As far as the double spend goes, I'm sorry that this happened, but blockchain is not liable for user initiated unintentional transactions.
So one thing I noticed was the size of the transaction directly impacted whether or not you would get a 502. Small (25-30 addresses) sendmany were always successful, anything larger up to about 75 addresses was hit or miss and anything above 75 addresses were almost always 502 + really did send the transaction. Glad you got it worked out.