There should be no difference. However because the mirror client is relatively new there might be bugs with correctly transmitting hook values. In that case these bugs should be reported and will get fixed. But the scripts are ran by the same engine as the standard mode.
You are not a VIP, Sponsor or Staff member. Please read the guide on how to use the mirror client, it clearly states the requirements and instructions.
This will not work as a proxy for the client. You will connect through the browser with a proxy to load the OSRS client but the OSRS client its connection will not be through the proxy. We will introduce proxying per tab in the future for the mirror client.
Impressive
We're about to update the client to be compatible with the new update to the mirror client MGI made, fixing most of the current bugs in the mirror client.