Firstly, the code you mentioned in the question has compilation errors i.e.
propUpdate.FieldToReferAccount__r = prop.FieldToReferAccount__c;
this above line should have given you below compilation error
Illegal assignment from Id to sObject(Your Object Name)
FieldToReferAccount__r is a relationship field hence you assign an object to it and not Id ...
Your this piece of code is causing the error:-
if( (accCont.Name != account_i.Name) || (account_i.Name != accOpp.Name) )
You are trying to access the value from list directly using accCont.Name or accOpp.Name but you cannot as Name exist on single instance of Account but List is group of accounts.
To access the value from a list, Use Get method like ...
Check these things:-
To add team members to an account: Read on users AND Edit on accounts
Who can manage the account team members depends on the users’ access. Account record owners and users above the owner in the role hierarchy can add, edit, and delete team members.
Your page layout of account team member determines which fields are visible and your ...