This franchise as a history of telescoping leg segements, it's too late to suddenly say its too hard to be done. There have also been plenty examples of roll pins in panels no thicker than a credit card or two, and the toys still hold up. Considering the product line and the price point, metal parts should not even be off the table either.
Its not that complicated to give the collar plate two or three folds into a V or U shape, and then run a peg or tab through them, to be an "accordion". The vf-31's body is basically already vertically held together like that, despite being much more disproportionate. it's even simpler though, to just turn that collar plate into two trays that slide under and over each other to double up as locking tabs on opposite ends and double the total thickness if strenth was a concern. If anything, collapsing the plate to reduce overhang in any way, and adding thickness and fastening points between the backpack and the body should only make the whole toy sturdier. Bandai regularly does far more fiddly things than this.
The crux of the problem though, as seen above, is that none of this would actually be neccessary if the proportoins weren't so arbitrarily inaccurate in the first place. Since they are going the inaccurate route, they could have made the proportions into anything to avoid the gap as well. In either case, the size of that gap didn't need to be there in the first place.