Starting from https://lists.gnu.org/archive/html/texmacs-dev/2021-06/msg00014.html, here is a possible macro for wish b) (leaving it as it comes out from Copy to->TeXmacs)
<assign|mult-pos|<macro|a|b|c|d|e|f|g|h|ii|llll|m|n|oo|<tabular|<tformat|<cwith|2|2|1|-1|cell-bborder|2ln>|<cwith|4|4|1|-1|cell-bborder|2ln>|<table|<row|<cell|<math|<arg|a>>>|<cell|<math|<arg|b>>>|<cell|<math|<arg|c>>>>|<row|<cell|<math|\<times\>>>|<cell|<math|<arg|d>>>|<cell|<math|<arg|e>>>>|<row|<cell|<math|<arg|f>>>|<cell|<math|<arg|g>>>|<cell|<math|<arg|h>>>>|<row|<cell|<math|<arg|ii>>>|<cell|<math|<arg|llll>>>|<cell|<math|\<cdot\>>>>|<row|<cell|<math|<arg|m>>>|<cell|<math|<arg|n>>>|<cell|<math|<arg|oo>>>>>>>>>