On $p$-adic Siegel--Eisenstein series II: How to avoid the regularity condition for $p$

In a previous paper, the authors showed that two kinds of $p$-adic Siegel–Eisenstein series of degree $n$ coincide with classical modular forms of weight $k$ for ${\mathrm{\Gamma }}_0(p)$, under the assumption that $p$ is a regular prime. The purpose of this paper is to show that this condition on $p$ can be removed if the degree $n$ is low compared with $k$, namely, $n\le 2k+1$.