[...]Other users have also reported this problem using MCX and other wallets [...]
I'm using the plugin with 3 wallets: Coinbase (coinbase.com), Bitgo (bitgo.co) and Market Coin Exchange (mcxchange.co) and only one is compromised. So it must be coinbase's security. Blockonomics also strikes me as a scam.
Why is your conclusion that something is wrong with coinbase ?
Coinbase can't protect you if a 3rd party gains access to your credentials (did you use 2FA ?).
The preferred way to accept BTC on a website is always to do it yourself. And if this is not an option because the effort wouldn't be worth the amount you expect to get, use a payment processor. But if possible do it without WP plugins.
Only very few of them have an security audit being done. Vulnerabilities in WP plugins wouldn't be something new.