I am preparing to begin development of a large, indoor bonsai mango, but I have come across a few holes in my research.

It is vital that the main trunk grow thick and dense so that it will be able to support itself and maintain its shape once mature/complete. The trouble is that most mango trunks I have seen are a bit spindly, at best, unless they have been grown in-ground for a decade or better.

My list of possible ideas are:
1. Propagate several polyembryonic mango seeds together and bind them so that their trunks fuse.
2. Feed root-growth hormone/root stimulant to a single seedling to suspend apic growth in favor of a stronger root system, followed by a sudden, robust growth spurt.
3. Slash the trunk vertically and let it heal - repeatedly - until it reaches the desired girth.

The problems I am facing are those of ignorance: Will the combined growth rate of numerous mango trunks stack (does an inch of girth on five fused trees equate to three inches of fused girth), or will they simply divide the growth between them and remain thin? Will root growth hormone even work on seedlings and, if it does anything, is it helpful or harmful ... and will it even have the sudden robust growth spurt I anticipate? Can a mango withstand major trauma like a long vertical slash the way other tropicals can, and would the wound heal even remotely quickly enough to make it worth the wound?

My intent is to groom it into an informal upright shape, with a maximum canopy height of 6', but an overal trunk length of 10' to 12'. But it will not support itself or hold its shape without a strong, wide base and a gradual taper along the trunk. To do this, I need it to develop a thick trunk long before it reaches its target length.

So, I guess my question is whether any of my ideas are any good, and why or why not. And, if none of them are any good, do you have any further suggestions? Thank you all for your help.