Your document is well argued. We certainly shouldn't blindly assume that just because the ACE vs just-send-8 issue was argued to death in the IDN WG, the trade-offs between the two approaches when applied to IMAs will automatically be the same as those for IDNs. (Though I can also understand that this group probably really doesn't want to go there again.)
Please note that John never proposed 'just-send-8'. 'just-send-8' is different from ESMPT UTF8ADDRESS.