If you have a measured 600m trail route at hand, a good session might consist of running out to the 600m mark at 10k pace and then turning and running 5k pace back (2:30 recoveries).
You could repeat this 5 or 6 times in place of a mile rep session just for variation. The total distance run is between 5k and 10k race distance, and so is the average pace. Having to turn around and run back faster than you started will stop you going out too fast and bring control and consistency. SG noted 4:55 pace for your 600m intervals - which is great for leg speed and stamina. This session is more about working the full range of energy systems in limited time.
That's a monster session. I wouldn't attempt it without a taper, and a couple of rest days afterwards. I would say this session would dominate your week - i.e. the 2 days before and after would be written off in order to taper and then recover from it. Build up your mile reps one at a time - a progression with some plateaus.
But different people suit different training. Marty Dent, 2:11 Austrailian Marathoner (28:50 for 10k) does 8 x 1km @ target pace, and Jo Pavey, 2:26 GB marathoner does 6 x 1mile @ 10k pace. The key is to not go beyond the current target race distance but keep the quality high.
However, Only way of knowing if a session is useful is to try it yourself.
I like your trail intervals - in my opinion, any 5 miler on the trails has the same training benefit as 6 on the road since the surface instability and core engagement is much greater, and you are kept constantly "on task" since the terrain demands greater levels of concentration. 600m is a good distance for intervals also, since mentally you can treat it as one lap of a track - and then hang on.
Your "mile reps" is also a good session, but I think if your recoveries were a bit longer (@ 2 minutes) you could be a little more consistent (with a range of 5 seconds instead of 8) and it would feel a lot easier (but, admittedly you are losing endurance benefits at the expense of consistency).
My favourite bespoke speedwork session is 5 x 320m sprints (@ 4:16-4:20 mile pace) off 3 minutes recovery, with the sprint session bookended by 2 x 1 mile @ slightly slower than tempo pace (@ 5:55 mile pace). You are getting the full range of energy systems (sprints - anaerobic and upper end aerobic - tempo, as well as working at improving your LT and V02 max. This session takes discipline - you nearly always run the first rep way too fast and then crawl the last! I always prefer interval to tempo BTW, but cruise intervals are good at making the transition from steady warm up pace to fast intervals.
I think your last post is as much about trying to prove to yourself that your goal is realistic as trying to defend your position to others.
I think most of the forum posts have been sceptical at best. A 150 mile fortnight ( as opposed to your "strong" 150 mile month) will be needed to crack 15 minutes comfortably.
A 19 minute 5k is roughly equivalent to a 41 minute 10k performance and a 15 minute 5k equivalent to a 32min 10k. Putting your 5k performance into context (most people find 10k times easier to relate with), do you still believe you can cut roughly 20% off your time over the next few years and move towards semi-elite level?
Your past statistics are impressive, but they remain just that. In the past.