Hey,
I've worked on integrations with a whole range of banks across Europe, and I will say that all the sandboxes are dreadful and out of date and banks won't give you access to anything near production unless you have an account with them.
For business/corporate accounts you can integrate directly with the banks own premium/treasury APIs, in which case you generally won't need PSD2 certificates, however it does depend on the appetite of the risk department of the bank.
Of the requirements you're looking for, webhooks are not that common yet. This is mainly because it wasn't within the scope of PSD2. Banks are starting to implement this though as a premium feature.
If you want more detail I can give you an overview of what you need to know - sounds like we might be able to help here at Payable.
If you want more detail I can give you an overview of what you need to know - sounds like we might be able to help here at Payable.