I have formed 270 from 280 brass (it was packaged wrong and would not chamber) make sure you lubed case inside and out(neck) w/ Imperial wax then check to see if it chambers first before loading. The shoulder will most likely be off as I had to set the shoulder back on the 280 brass to fit the 270 chamber. To do this seat the bullets long enough to jam the lands and use a mild load to fire form.
A 280 Rem case is slightly longer than a 270 Win so that it isn't accidently chambered in a 270 rifle. That would create a pressure disaster trying to get the .284 bullet down the .277 barrel. I would bet the 270 case would go through the 280 die like butter but the slightly shorter shoulder would have to be fire formed to get the full dimension of the longer 280.
I believe so. Just don't fireform with a real hot load and you should be okay. Try 10 or 20 and see what happens. Fairly new good brass should just stretch right into shape. Older brittle brass could do a case head separation because of the extra headspace. If this worries you sell the 270 brass and buy some 280 brass, shouldn't cost you much and saves the hassle.
Why not anneal the cases, run them through the fl 280 die as you normally would, seat cheap bullets out to touch the lands (no headspace issue this way) use a slow for bullet weight powder about 1.5grns under max with like cci 200 primers fire them and bam, a brand new batch of 280 cases with a stronger case web to boot, because I bet the 270 brass is thicker and heavier at the case head and up past the web due to the 270 having a much higher pressure rating. To each his own, be safe.