I agree with your points raised. However, I think that the form can be managed at admin level in each business using form defaults, limiting the users of that company to only settings that they would actually use.
I do agree that as new functionality is added to Manager, admins may not be aware of new settings in form defaults. But I think that this is a design flaw of manager that could be fixed with an alert to manager (similar to the yellow alert when there are one or more recurring pending to be issued). They could be made aware when they login, that there are new settings to review. The same complaint could be made about other new settings in Manager. I am not alerted when there are new options. But this could be resolved quite easily.
I think the real problem is that the design of the forms is not optimal as it shows all possible options regardless of whether a business whether ever use a setting such as total amount in words. What AMM suggested is a possible way forward, but personally I just think that this is treating the symptom of the problem. Speaking for myself, the only box that I have ticked is the custom theme, which I have set in form defaults anyway.
To me, the attributes actually serve no purpose (as I am already using form defaults to set the attribute) and the only time I change a setting on the forms, is to see what it does! I would suspect that the vast majority of users are like myself, that they set the attributes once in form defaults and then never ever change it in future.
What I would suggest is a form default setting in Manager to hide/show each attribute on the invoices forms that is actually being used by that business. Those settings should be set by the manager who has access to Manager settings.
Thinking about it, I am a bit surprised that any user that has access to settings such as hide due date or total amount in words. I would consider than an admin level setting personally.
I am not saying that this is the way forward and I don’t really agree that I am hiding settings if the admins are always made aware of new settings that could be applied. It’s just allowing the managers of each business to decide what end users see as most of the attributes will never be used by that particular company. I only use one setting! Hopefully @lubos would consider this as an option.