Complex Payment schedule

My formula query is:

I have two different payment streams. One from 01 January to 31 December and the other from 01 July to 31 December.
Any client (Column A) that has a START DATE (Column D) prior to 30 June is paid at one rate and if any client has a START DATE from (on or after) 01 July it is a different rate – that part is reasonably simple for any new clients. However –a client may have a START DATE previously in a different UNIT (column I) but the same SESSION CODE (column H), then the rate needs to default to the earliest date displayed in Column D. I’ve tried to put some workable logic to help anyone to understand the mathematical formula.

If both Client ID and Session are the same, (even though the UNIT may be different) then check the start date of the first occurrence, and if pre June 30, the outcome could be “PRE”, or if post July 01, then the outcome could be “POST”.

Scenario:
First check the client ID if it appears anywhere else, if not then use the start date on this line and populate as “PRE” or “POST”
If the client id is found, then check for the Session Code, and if not a match then use the start date on this line and populate as “PRE” or “POST”, I can then implement a lookup table to determine payments based on the outcome.
If the client id is found, and the Session Code is found, then use the earliest start date and populate as “PRE” or “POST”.

If the client id is found multiple times, then check for the Session Code, and if a match use the earliest start date and populate as “PRE” or “POST”.

A good test Client ID is 122229640 (use filter on Column A), I’ve highlighted the dates.
This client should be paid at the “PRE” rate for all, including those listed in June, September, August and October as the first date of occurrence in both sessions is prior to 30 June.
This is a rather complex formula, and it can be placed in Row “AB” of the PAYMENTS Sheet. I have hidden some of the other criteria I use to determine a very complex payment system that has just had another curly component dropped into the mix.

I do hope someone can assist.
Thanking you in advance.
Chris