Sorry to be slow responding to your great question. You’re thinking about your preparation in the right way. Having to clear paths to choose from will make the decision easier. Thanks for articulating them so clearly.
I’ve had more athletes do well coming off a heavy training block into a taper than I have coming off B races into a taper for the main event. However, the B race option has real value and I understand the appeal. As you mention you can hone strategy, fueling, hydration, self care etc in a supported race better than in an unsupported training run. That’s a big plus and needs to be weighed heavily.
Why I lean toward the training block/training camp approach is that there is less pressure and the training can be better controlled. I also feel that it is better to spread the training load out for several days allowing your body more time to absorb and adapt to the training stress, rather than slam it with a hard race effort. However, I’m not sure I’d go for a 9 day concentrated block unless there a couple of rest days built in.
If you decide to do the B race approach what about doing only one B race and a shorter training camp? If you go with the B race approach then I think you really need to run this race as a training run not pushing into the red zone during it. It would be very easy to leave your race on the training ground. I’ve seen this happen all too often. A concentrated effort like a 50km race is going to take many days to absorb and benefit from before you are ready to get back to actual building training.
I believe having the confidence of knowing that your training has been spot on leading into the race will be as valuable as doing the B race (I’m quite dubious of trying to fit 2 into March for an April A race0.
For a races with this much vert I have had great luck implementing a protracted ME block during the “Base” phase. I’ve had great using exactly our gym ME progression described in TftUA 1x/week. I’ve done this block for up to 16 weeks. During this the emphasis is on maintain aerobic base with medium volume abut ALL of it easy. Lots of Z1 and recovery work with very little Z2 unless you have ADS. Then the focus shifts to more event specific training during a final “Specific” phase. In this phase we use conventional interval training methods, usually also only 1x/week and the volume begins to build. My favorite way of doing this is with back to back long runs on weekends.
Both the Training Camp model and the B-race model rely on the “Over Reaching” principle. Over reaching is a valuable training tool for advanced athletes who have a lot of experience and are good at judging when ‘enough is enough’. In a race you may not have the presence of mind when you have stepped over the line and enough becomes too much. In the Training Camp model you must be careful to not become a slave to an overly ambitious plan. Over “over reaching” in either of these scenarios can spell disaster. I’d much rather have an athlete undertrained than overtrained going into a race any day.
I know there is nothing concrete here but I hope my musings help your thinking.