BMW highly recommend you
do not change the transmission oil on the DCT. Due to the way they work there is a very real risk of a short fill.
As for the manual it's personal preference. BMW/MINI are on a crusade to reduce the amount of services a vehicle needs in its lifetime/100k miles. It's in their interest to convince you of the idea of 'lifetime' oils. As such they will never cover lifetime items under warranty or service plans.
That said, I've build more than a few engines and gearboxes and I do not believe in such mystical things as service free gearbox/fuel filter/engine oil.
It's entirely your call but my dealer charged me £48 for the gearbox oil - but I missed a gear change while being a little to enthusiastic
Who wants small pieces of metal in their gearbox oils for 100k miles?
I drive my cars HARD so change my fluids every 8k rather than the 16k on the service plan. If you've only going to keep it three years rag the fuck out of it, do hard pulls and let the other guy worry about it post warranty. If you have even the slightest inclination to keep it my vote is to look after it.