The square / single-glyph version is only used in some / limited contexts, in most contexts (including the emperor's eventual posthumous name) heisei would be written and displayed "normally".

I'm not competent to answer that, but the "merged glyph" versions of the other four modern era names date from older encodings like EUC and shift-JIS, and are presumably in unicode for compatibility. So the rationale for adding a new one may just be extending that set.