It's not unusual for folks to need several tries on different terrain (sustained climbs, long flats, etc.) before actually achieving their FTP for a full hour. Remember FTP represents your best possible power for a full hour under ideal conditions not your on demand hour power any day of the week, any issues with recovery, blocked up legs from too much rest, poor nutrition, hydration issues, poor pacing during the test or insufficient motivation to really give your all for a full hour can all contribute to falling short of your potential. It's one of the reasons so many folks use shorter tests including Monod testing to estimate their FTP, it's just really hard to nail your best possible power for a full hour out of competition and even during an actual 40K time trial it can be tough if everything doesn't click on race day.
I have achieved very similar results for my maximum one hour power on three occasions.
I had a couple of good attempts at achieving my FTP up Mt Buffalo and the results were within a few watts of each other. I had a positive Chronic Stress both times, but my legs may have been a bit blocked up as I had waited three days for the rain to stop. It took me 62 minutes and I was trying hard to crack an hour.
I defiantly had a pacing problem, as at the 10 minute mark I had averaged 281 Watts, but at 20 minutes it was 276 Watts, at half an hour it was 271 Watts and by the end of the hour it was down to 266 Watts.
FWIW, I've never seen my best full hour on a long climb, first it's hard to find a full hour of climbing without flatter sections along the way but for whatever reasons I tend to do better for a full hour on the flats even though many of my best 5 to 20 minute efforts have been set on climbs. Everyone's different,but you might try some long efforts on different terrain to see what happens
I have never had a crack at doing a 1 hour FTP test on the flat. The terrain around where I live is a bit on the rolling side, which is one of the reasons why I have done my power testing indoors or on long ascents.
You've also got the issue of Monod numbers set indoors on the trainer vs. outdoor riding. Many if not most folks can sustain different power indoors vs. outdoors though your recent 5 minute hill climb MMP suggests that it might not be that different for you assuming that was a maximal 5 minute effort.
I was aware of this, but I have found pretty good correlation with my power numbers on the road, hills and the indoor trainer. I have however noticed an inability to reach my MMP curve in time trials, which may be part of the reason why I am so uncompetitive at them. For some reason, I can't time trial as well as people of similar weight who I can out climb.
I'd probably stick with your 280 estimate for FTP but it doesn't really matter that much. The key thing is to target appropriate durations for your intervals like 3 to 6 minute for VO2 Max work, 20 to 30 or up to 60 minutes for Threshold work and just ride them very hard but backed off enough to finish the efforts and finish the sessions. The appropriate levels will become really clear after a session or two and things will self correct very rapidly. If for instance you can't finish a 20 minute effort at 255 watts or more then your FTP is likely set too high, if you do a set of two or three and hold 280 or more for all of them without feeling you worked too hard then bump it up a notch next time. You'll know real soon what you can manage and hopefully those levels will change with additional training anyway.
I have gone with the 280 estimate. After reading this, I did a 2 x 20 minutes session. The first was done at an average of 280 Watts and after 10 minutes rest I commence the second. I was feeling pretty comfortable at the half way mark, having averaged 280 Watts to this stage, so I upped the wattage until I felt uncomfortable and I ended up averaging 285 for the 20 minutes. If I was to have done three sets, 280 Watts would definitely have been achievable