Thank you for your request.
I have the answers for your questions here below:
1. How can I send an event to GA for failed transactions?
This is always compatible and has nothing to do with our plugin.
Google Analytics does not interfere with the payment process - they only access Shopify components (such as "Confirmation Page).
So you should be able to find more informations through Shopify.
2. How can I test failed transactions? What steps must I take to create a testing environment and fire failed transactions?
This depends on your PSP. There is for example a "Simulate failure" button on the payment page in test mode.
3. How does the Failed url look like?
I don't know exactly - that's what Shopify does / it's the standard Shopify process -> it's best to contact them directly.
In any case, the customer is redirected to the checkout with an error message as standard, so that he has the fastest possibility to pay in another way -> explicit Failure Pages are for this reason rather untypical.
I hope that I could help you out with this.