Is it possible to apply a diacritic or combining enclosing mark to a sequence of more than one (non-combining) character?
No, with the exception of the “double diacritics” deliberately designed to be applied onto a two letter sequence, e.g. U+035D COMBINING DOUBLE BREVE. Neither ZWJ (U+200D ZERO WITDH JOINER) nor CGJ (U+034F COMBINING GRAPHEME JOINER) “glue” characters together in a way that the scope of any following combining character would be affected. To get a character sequence like “Esc” into something like the U+20E3 COMBINING ENCLOSING KEYCAP, you must resort to higher-level protocols.