Restricting People Control Choice to a Group with a Condition

Nintex Forms 2013
Windows 7 - IE 11

Problem 1

I have:

• Field A - Choice control with 5 choices, Choice1, Choice 2 . . .
• Field B - People control, with Appearance setting of "Expression" - set to Is_New_Mode
• Group - Group XYZ

I need to add:

1. Restrict Field B - People choices to a Group, similar to using "IsMemberofGroup", but the Group is chosen based upon the value chosen during runtime from the Field A - Choice control
2. Example: 
   1. Field A - selects "Choice 2"
   2. Field B - People choices can only be from "Group XYZ"
   3. If Field A selection is <> to "Choice 2", then there is no Group restriction
3. The validation could occur at the time the item is Submitted/Saved, with an appropriate validation error message, but obviously would be nice to have the validation occur during field entry.

Also, conversely:

Problem 2

I have:

• Field A - Choice control with 5 choices, Choice1, Choice 2 . . .
• Field C - Calculated Value that contains expression "userProfileLookup(Current User,'PreferredName')
• Group - Group XYZ

I need to add:

1. Restrict Field A - make the selection of "Choice2"  allowed only if the Current User is a member of "Group XYZ"
2. The Current User is able to select any other Choice <> to "Choice2" whether they are a member of "Group XYZ" or not. 
3. Example: 
   1. Current User has opened a New Item Form, and their "Preferred Name" is captured in Field C
   2. Field A - user wants to select "Choice2", but is evaluated and validated as to whether they are a member of "Group XYZ"
   3. Field A - user selects any value other than "Choice2" without restriction
4. The validation could occur at the time the item is Submitted/Saved, with an appropriate validation error message, but obviously would be nice to have the validation occur during field entry.

Runtime Rules do not have an "IsMemberofGroup" option; and I wasn't clear if I could accomplish this from a conditional statement under the control settings for Appearance.

Not quite like a Cascading choice control, as the People control does not have defined choices (other than the group of course)

Adding an afterthought - Is there a solution/best practice for creating Runtime Validation for People/Groups, not necessarily with a condition, or is that simply the use of the Appearance setting for "IsMemberofGroup" ?

Thanks

Mark