Why does the Malayalam script have seemingly redundant ways to kill implicit vowels?

Question

In the Malayalam script there are two ways to kill implicit vowels. The most widely applicable is the chandrakkala diacritic, similar to the virama that appears in other Brahmic scripts. There are also chillaksharam, modified forms of a small subset of consonants. Interestingly enough, the current sounds of chillaksharam don't match the consonants they appear to represent, hinting that a phonetic shift occurred since their introduction.

Why do these two ways of killing implicit vowels coexist? Do the chillaksharam have a Dravidian origin, or do they come from distinct attempts to adapt Dravidian scripts to accommodate Sanskrit vocabulary?

Answer 1

The Chillu is a modern reform, primarily supposed to be used at the end-of-word to explicitly represent a killed consonant, whereas the Chandra mark is used to denote semi-killing; that is, depending on the dialect, it produces [ə] or [ɯ̽] or [ɨ] sound.

Example:

Malayalam	ISO 15919	English
ആ മനുഷ്യന്‍ കൊടുക്കുന്നു	ā manuṣyan koṭukkunnu	That man gives
ആ മനുഷ്യന് കൊടുക്കുന്നു	ā manuṣyanŭ koṭukkunnu	Give to that man

Note that "മനുഷ്യന്‍ " means "man" and "മനുഷ്യന് " means "to (the) man".

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Why do these two ways of killing implicit vowels coexist?

So in general, any final-candrakkala should be transcribed as half-vowel, and intermediate-candrakkala as glottal stop. Chillus should ideally be used only wordfinally. Hence no ambiguities exist if the writer follows the rules.

This confusion is because historically, u+candra symbol (e.g. നു്) was used to explicitly denote the half-vowel and candra was used as a pure-virama (e.g. ന്). But printing standards converged to candra-form for both the cases causing ambiguities (more details here), hence requiring a new reform (e.g. ൻ encoded separately) to represent some wordfinal dead consonants (more details here).