You could load packages scrlayer and scrhack. Then you can declare a layer containing the page number and a layer pagestyle for included landscape pages.
\documentclass[10pt]{article}
\usepackage[margin=1in]{geometry}
\usepackage{pdfpages}
\usepackage{pdflscape}
\usepackage{scrlayer}
\usepackage{scrhack}
\DeclareNewLayer[
background,
textarea,
addwidth=\footskip,
addwidth=\footheight,
contents=\hfill%
\rotatebox{90}{\parbox{\layerheight}{\centering\pagemark}}
]{lscape.foot}
\DeclareNewPageStyleByLayers{lscape}{lscape.foot}
\begin{document}
\section{Test Section}
Test Vertical, number bottom
\clearpage
\begin{landscape}
\includepdf[pages=1,landscape=true, scale=.9,pagecommand=\section{Task Sheet}\thispagestyle{lscape}]{example-image-a4-landscape.pdf}
\includepdf[pages=1,landscape=true, scale=0.9,pagecommand={\thispagestyle{lscape}}]{example-image-a4-landscape.pdf}
\end{landscape}
\end{document}
Result:

You can also use package typearea to change the paper orientation:
\documentclass[10pt,
letterpaper% <- paper size as class option
]{article}
\usepackage[usegeometry]{typearea}% load before geometry
\usepackage[margin=1in]{geometry}
\usepackage{pdfpages}
\newcommand*{\useportrait}{%
\cleardoublepage
\KOMAoptions{paper=portrait,DIV=current}%
\newgeometry{margin=1in}%
}
\newcommand*{\uselandscape}{%
\cleardoublepage
\KOMAoptions{paper=landscape,DIV=current}%
\newgeometry{margin=1in}%
}
\begin{document}
\section{Test Section}
Test Vertical, number bottom
\uselandscape
\KOMAoptions{paper=landscape,DIV=current}
\newgeometry{margin=1in}
\includepdf[pages=1, scale=0.9,pagecommand=\section{Task Sheet}]{example-image-a4-landscape.pdf}
\includepdf[pages=1, scale=0.9,pagecommand={}]{example-image-a4-landscape.pdf}
\useportrait
\section{Second Test Section}
Test Vertical, number bottom
\end{document}
