I'd like to ask you to reconsider procedure of friend list purging. Even though as you said "This is a problem of Steam not updating their 'cache' or whatever of the friendlist.", actual number of friends can be updated by sending friend request on your own, after another person accepted invite, number of friends gets updated. Thus I'd like to ask completely remove all friends and readd owner of the bot when purging is necessary.
And if it's possible, could it be done on my bot because no one can add it atm... ;(
Thank you for your time.

This will no longer be an issue later (as we will change the trade method to sending trade offer, no friend adding is needed).

I have tried to send friend request from the bot anyway to clear that 'cache'. It seems to have worked. :)