>
>r> I usually take 'sabbamangalam' as meaning blessing to all, as in
>r> 'bhavatu sabbamangalam.'
>
>Why? It is third person singular imperative,

In itself that's not a reason against it. "Let there be
blessing(singular) for all". But then you provide more context in the
below which could change it. Still, just taken by itself it seems to
work.

>Bhavatu sabba mangala.m,
>Rakkhantu sabba devataa,
>Sabba Buddhaanubhaavena,
>Sadaa sotthi bhavantu me.
>
>May all blessing (good fortune) be
>...
>... for me.