Since some of the data is derived from the Merit Dashboard, I guess I can provide the answer here to some of your questions:
<…>1. Did you use the current rank of users or the rank of them at the time the merit transaction was done?
The data displays the current rank (as of last Friday) of both sender and receiver. That does mean that, when one ranks-up, he drags along the awarded or received merits to his next rank in a block (so as to say).
2. Look at the first row of your table. The percentage below Sr. member. That's 11.11%. What does it mean? Is 11.11% the percentage of merits Sr. members have sent to Legendary members or that's the percentage of merits Legendary members have sent to Sr. members?
The numbers represent what % rows have sent to columns. In the case you mention, Legendries have sent 11,11% of their sent sMerits over to Sr. Members (note: rows do not add up to 100% due to some other ranks not accounted for here – i.e. Administrators, Mods, VIP, etc.)
3. 11.11% of what? 11.11% of all merits sent by Sr. members (or received by them)? Or 11.11% of all merits sent by legendary members (or received by them)? Or anything else?
Sent from Legendary to Sr. Members, as mentioned above.
4. Shouldn't total percentage of one column or one row be 100%?
Yes, but I figure that, in order to simplify, the percentages of the non-regular ranks are ommited.