Emoji Space

[Christoph Päper via Unicode]

As you may know, the combined original Japanese emoji set included three whitespace characters: one was the full width of a (square) emoji, one was half that and the last one was a quarter blank. Their KDDI Shift-JIS codes were F7A9, F7AA and F7AB, respectively, and their internal numeric IDs were #173, #174 and #175, respectively. They were apparently not adapted as new Unicode characters and no existing space character gained the Emoji property. 

Which existing characters then should be used to align emojis (in a square grid)? 

Since emojis are square glyphs in all relevant implementations one would expect at least an em-wide blank would be available in emoji input systems. Is this U+3000 Ideographic Space, U+2003 Em Space or U+2001 Em Quad?

I assume the other two original emoji spaces are best mapped to U+2002 En Space and U+2005 Four-per-Em Space. There is no white space character explicitly half an em wide, it seems.

Finally, should smart fonts make U+0020 exactly as wide as an em when between two emojis?