Sorry to be such an American, but what is 10-20 C in degrees farenheit?

You could do a trial run of swim-bike-run in the swimsuit/bike shorts combo. If the seams aren't going to rub you the wrong way, it should be fine. I certainly see lots of people riding and running in swimsuits.

I must wear a bra, so I wear a bra + Sugoi trisuit + t-shirt for the ride/run if I'm doing an indoor tri, or bra + trishorts + wetsuit, for an outdoor swim, peel off wet suit, pull on t-shirt, and I'm off.

I definitely would not plan any outfit that requires taking off the underlayer. Wastes too much time in transition to have to switch outfits like that.