Fallback for Sinhala Consonant Clusters

[Richard Wordingham via Unicode]

Are there fallback rules for Sinhala consonant clusters?  There are
fallback rules for Devanagari, but I'm not sure if they read across.

The problem I am seeing is that the Pali syllable 'ndhe' න්‍ධෙ <U+0DB1
NAYANNA, U+0DCA AL-LAKUNA, 200D ZWJ, U+0DB0 MAHAPRAANA DAYANNA, U+0DD9
KOMBUVA> is being rendered identically to a hypothetical Sinhalese
'nēdha' නේධ <U+0DB1, U+0DDA DIGA KOMBUVA, U+0DB0>,  which in NFD is
<U+0DB1, U+0DD9, U+0DCA, U+0DB0>, when I use a font that lacks the
conjunct.  (Most fonts lack the conjunct.)  The Devanagari rules and my
preference would lead to a fallback rendering as න්ධෙ  (Sinhalese
'ndhe'), which is encoded as <U+0DB1 NAYANNA, U+0DCA AL-LAKUNA, U+0DB0
MAHAPRAANA DAYANNA, U+0DD9 KOMBUVA>.  Is the rendering I am getting
technically wrong, or is it merely undesirable?

The ambiguity arises in part because, like the Brahmi script, the
Sinhala script uses its virama character as a vowel length indicator.

Missing touching consonants are being rendered almost as though there
were no ZWJ, but the combination of consonant and al-lakuna is being
rendered badly.

Richard.