In (1) you assume that people buy green energy mainly or to a large extent to reduce the emissions they personally suffer from. Have you considered that a big reason for many people to buy green energy might just be to provide incentives for green energy production in order to combat emissions globally without regard for whether they personally benefit from it? If everyone consumes energy "covered" by certificates, this increases viability of producing green energy.
In (2) you say that the scheme is likely to be gamed. Can you elaborate on how that would look like? Does this "Occam's razor" necessarily mean that only simple laws are effective?
Also in (2) you claim that there is not enough real green energy available to satisfy the demand. This is obviously true but this means that the demand creates an incentive for companies to produce green energy wherever possible.
Overall what this system really provides is an option for consumers to optionally subsidize green energy producing companies and so far I have not really heard of any concrete case in which this system has been gamed. Do you know any?