Event Registration Reminder issues

Problem/Motivation

We are using recurring events registration and recurring events registration reminders submodules. I have noted two issues and I'm not sure why we're seeing what we're seeing:
1. An event series is created with registration reminders turned on. Next, an event instance is created. Until the event series' registration reminders schedule is changed, the event instance's reminder_date stays null.

2. If the event series' reminder registration schedule is changed (e.g. from '1 day' to '2 days'), ALL event instances have their 'reminder_sent' value set to 'NULL' which means past events will receive reminder emails.

Steps to reproduce

- Create a new event series
- Enable regisration, and registration reminders.
- Create an event instance
- Note that the table 'eventinstance_field_data' has 'reminder_date' equal to null
- Update the event series and change the registration reminder schedule
- Note that 'eventinstance_field_data' now has 'reminder_date' set, but for ALL instances, even those in the past, 'reminder_sent' is null.

Proposed resolution

I'm not sure how best to approach the first issue. The second is fairly straightforward - it should validate against the event end date before setting the 'reminder_sent' to null.