Reflexive coequalizers are examples of sifted colimits, and groupoids are examples of reflexive coequalizers. But quotients don't preserve monomorphisms in general.
For example, in the category Set (considering question (2)), let $V = S \rightrightarrows S$ denote the trivial groupoid acting on a set $S$ with two elements, and $W = S\times \mathbb Z/2\mathbb Z \rightrightarrows S$ the action groupoid of $\mathbb Z/2\mathbb Z$ acting on the same set $S$. There is an inclusion map $V\to W$, which is injective on both objects and morphisms. But the induced map on the quotient sets is not injective.
I am not sure what this says about the categorical version, but I think it indicates that the sifted case is very different to the filtered case.