would it work to gently microwave them in a silicone mould so that you don't then have to re-shape? Or is that a crazy idea?
Not crazy, maybe just not thought through all the way. Consider that microwaves heat from the inside out, so you would have uneven melting rather than uniform melting. You would need to stop and stir through several times to get it uniform, in which case don't bother with melting in the mold.
I decided to try crumbling the bar and melting. I also did a weight calculation to volume, even thought I don't have mango butter I do have shea butter which has a "close enough for me" density to mango butter, so I melted the shea butter to see the volume and compared that to equal weight of the walnut oil that I used, and added enough weight to the walnut oil to equal the volume measurement of the shea butter. I'm not sure that really made sense, but as it did seem between my two experiments that the one with butter mixed more correctly than the liquid oil, that being short the liquid oil as a binding ingredient was more the problem. Water does not seem to be enough of a binding ingredient once it evaporates off as those bars spontaneously broke. So for the bars that I made I was short approximately 6g of liquid oil by weight to equal the same volume. This probably isn't a good/preferred formulating technique, and more sciency people than me will probably debunk it... but that's what I did!
The bars crumbled extremely easy with my fingers, so that wasn't the mess I was anticipating. I added the additional 6g of walnut oil, and then zapped it in the microwave for two 30 second increments. I probably should have done one 30 and one 15 and that would have been plenty. It melted very nicely together. I probably could have skipped the additional liquid oil I think, or perhaps should have waited to see how it melted out. Would have saved me a bit of time if I had thought of it. I did need to let the mixture cool, it was extremely moldable (and fun to play with) but had a tendency to ooze out of its shape. I had a bit of fun playing with it while it was cooling
It molded very nicely, the first bar I did molded very smoothly. The second bar has a bit more texture and lines as I did scrape the drier harder bits out of the melting bowl into the press mold before shaping it.
So what I learned from all this is that for the More Mango recipe, it is probably better to apply heat to get it to a moldable consistency rather than adding water. I don't think one would need to melt fully in that case, just enough to give the mixture a bit more "sticky together". I don't think I would worry then about adjusting the oil quantity at all. I should note that I did not add more fragrance or preservative to my remolded bars - even though I probably did kill off the preservative. They still smell great! These will be going to the boys shower and at the rate they've gone through the other shampoo bar, I don't think they'll have time to grow any nasties. If these were going elsewhere, I would have added more preservative once the mixture had cooled.
So far all of my testers have come back with positive reviews of this formula (minus the comments regarding breakage and crumbling, which I think this experiment has resolved), so once I confirm with this round of bars that I have figured out the source of the mixing/crumbling issues and the correct fix, and compare to the C14-16 bars (which I haven't made yet, our second son graduated from high school over the weekend so I did not have as much "play time" as I would have liked)... I think the "more mango" bar is going to be a winner for me.
I've only had one random crumbly bar at the beginning; after that, the rest have turned out great. I've started using my mooncake press for more uniformity, with fewer crevices and uneven spots - much nicer than anything I can get from hand-pressing it into silicone molds. 
