The councellor and DP were encouraging me to confront FIL when he said something I found offensive and tell him calmly about my own 'reality'
WTAF???!!!
Your dp's f has elevated himself to the positon of Supreme Intelligent Being who adores sounding off the sound of his own voice, and the counsellor and your dp cosy up together and tell you to confront him if he says something you find offensive?
The counsellor saw fit to task you with a specific exercise; what exercise was your dp tasked with? Did the counsellor suggest that, in the event his f ran true to form, he support you or were you expected to slay the dragon put yourself in the line of fire with no back up?
It seems to me your counsellor's out of their depth and, in dismissing your legitimate and understandable concerns in relation to your dp's loyalty to you and possible future dcs, it would appear that they have an eye to repeat business at some future date.
It's not difficult to understand why your dp has grown up to be a people pleaser and I would suggest that you seek individual counselling to work out whether you are prepared to make the necessary compromises to remain in a relationship where you cannot necessarily count on his loyalty to you in the face of any further/future onslaught from his f.
That said, if you're prepared to grow a thick skin, see your dp's f as the bombastic, overbearing, but ultimately harmless, buffoon that he is, and let him sound off without rising to his bait, I don't see this as an insurmountable problem.
As he grows older you can treat him as if he's in his dotage, pat him on the head, and patronise him with comments such as 'how lucky we are to have someone as knowledgeable as your good self to show us the error of our ways' while smirking quietly to yourself.
Practice tolerance, honey. It makes the world go round and pours oil on troubled waters.