I had a similar issue on my '96 855.

Flare nut was not abused (like on yours), but it was siezed in the caliper and no amount of penetrating oil, or babying, would free it. Realizing that this thing was not going to come out nicely and, realizing that I was going to have to replace the brake line anyway, I cut the line and worked on the caliper and flare nut on a bench. Even with that level of attention, the flare nut broke in the caliper and I ended having to drill it out. That was almost successful except for a few of the flare nut threads that absolutely refused to release themselves from the female threads of the caliper. I did not have a metric tap of that size and nothing I could do would unsieze those bits of metal in the thread.

Ended up having to replace the caliper.

Since you are going to have to replace the brake line anyway, I'd cut the line and work on removing the flare nut on a bench.

BTW, you should always use a flare nut wrench on a flare nut, not an ordinary opened end wrench. But with the brake line cut, you could use a boxed-end wrench which might give you purchase on the abused fitting.