For the interested reader, loading natbib before the template already solves my section title problem.
Also commenting out \bibliographystyle{IEEEtran} in the template solves it. Though I am a bit concerned about the journal using the original .sty file.
The issue has been resolved in version 3.5 of biblatex. (At least for all standard styles. Custom styles may need further tweaks, see below.)
If you are still having this problem, please update your TeX distribution. If the issue occurs with a custom style and does not go away after updating, notify the style author.
If you are using an online service, note that Overleaf v1 still uses the now quite outdated version 3.4 (2016/05/14) of biblatex. Overleaf v2 has biblatex 2016/12/05 v3.7, where the issue should be resolved.
biblatex's new context-sensitive delimiter system is a bit confused by the two-pass structure of the \textcite macro (which internally calls the command \cbx@textcite to do all the work), you will need the line
\DeclareDelimFormat[cbx@textcite]{nameyeardelim}{\addspace}
to help it along.
As PLK notes in the comments this issue (#432) has been resolved for all the standard styles in version 3.5 of biblatex. If you use a custom style that uses the two-pass \textcite but has not been updated yet, you might need
\DeclareDelimcontextAlias{cbx@textcite}{textcite}
instead.
MWE
\documentclass{article}
\usepackage[style=authoryear-icomp, natbib=true]{biblatex}
\addbibresource{biblatex-examples.bib}
\DeclareDelimFormat[cbx@textcite]{nameyeardelim}{\addspace}
\begin{document}
\citet{sigfridsson}
\citereset\textcite{sigfridsson}
\citereset\citep{sigfridsson}
\printbibliography
\end{document}
Best Answer
You can use citet combined with numbers citations by loading: