limit number of records user can have on a specific table
I'd use tablename_init() to set a count of records into a hidden field, then use tablename-{tv,dv}.js to remove the "Add New" button as needed. You will, of course, have to have some sort of user record - or co-opt a field in the membership info - to store the table + maxrecords info.
Or, you could do it all in the js file(s) by using an ajax call to get the reccount for the table, and then use jquery to disable/remove buttons etc as desired.
email notifications
Sending the email should be easy, so I assume you mean developing the logic to decide when/what to send?
I don't want user to be able to change once verified
I do something like that with one of my apps. Once a record is "closed" (a checkbox field), when the record is rendered thereafter ALL the fields are disabled. It would have been easy enough to selectively disable some and not others.
Also, when they "close" a record, I create an entry in a history table that retains a copy of the record. If the admin later opens the record so it can be edited, and then it is closed again, a second copy of the record is written. This provides a simple audit trail of "closed" copies.
Note that this is done with javascript and ajax. Safe enough from the average user, but by no means secure. To decrease the chance of hacking, it would be safer it it was all done in PHP. (On the other hand, in a few years of real world deployments, nobody has thought to twiddle the javascript.)
'upgrading' log-in to more robust secure system
I'll let the AppGini folks address this one. My apps are used internally and don't face the Internet, so it's no a major concern for me.
any limitations on use (number tables, groups, users)
I've never exceeded 30 tables, and my user and group counts are never more than double digit. I have generated tables with over a hundred fields in them without any issues, and that's running on a CentOS virtual server hosted on a Windows 10 system (ie, not exactly a speed demon!)
Maybe someone else has tried to hammer it harder than I have?
Tab Forms & Day/Time calculations
Glad to help!
I'm eyeballs deep in trying to get a grasp on a project that was done in Codeigniter, so I'm not spending as much time with AG at the moment. But, if I see something I can help you with, I'll certainly try.