I am trying to create a new variant metafield with the key ‘mm-google-shopping.id’ that is recognised by Google Merchant Centre, however the error message I keep getting is saying that ‘key must be a minimum of 3 characters’ - is there any way I can reduce this to 2 characters instead for this purpose?
I recommend contacting Google for support: https://support.google.com/merchants/contact/shopify_3p_support_app
Based on your query, I assume you want to change the product ID using your own values, not predefined by app.
In this case what you can do, is use a different data feed app, and for example use your SKU as your data feed value. This is what I use for my clients: https://apps.shopify.com/multiple-google-shopping-feeds
You could also submit a custom attribute, and then remap it using feed rules in Google merchant center.