On the Merchants Page, the following payee names and transactions are not accounted for:
• The underlying payee of a transaction that is linked to a Recurring Item
• The visible payee of the recurring transaction (the Recurring Item's payee name)
• The payee of a transaction inside a transaction group
• The payee of the parent of a split transaction
Since you can update the payee of a recurring transaction by updating the payee of the Recurring Item, it should also be possible to update it via the Merchant Page. Ideally, it should also be possible to update the payee name of the original payee name associated with the transaction that is linked to a Recurring Item (currently not possible from the Transactions page either).
Currently it's already possible to update the payee name of transactions inside a transaction group, so it should also be possible to do so via the Merchants Page.
The payee of the parent of a split transaction cannot be changed currently without unsplitting first. Ideally, it should be possible to update the payee name, both via the Transactions page and the Merchants page.