Probably would like to rephrase the requirement a bit to make it clearer to understand.
From case 1 I assume that you still wish to keep credit limit check - does it apply for both cases or it should be valid only for some customers?
In case you wish to have credit limit check only for some customers you will need at least 2 credit risk categories in a standard system, so that you can have different settings in OVA8 for the same documents.
Regarding consideration of open items - you can explore number of days for open items setting in OVA8. It is performed against the already available open items, which do not have a special g/l indicator not relevant for credit (FBKP transaction).
Your client may be also interested in overdue items check in addition to open items check (most companies do) and there is a way to define the number of 'grace days' for overdue items if this is allowed by the company policy.