You should have in your list: Devuan chimaera Devuan chimaera-update Devuan chimaera-security Leste chimaera Leste chimaera-devel And for apt-get invalid signature, sorry never seen error like that, indeed something is probably wrong in your sources list. And yes you can safely use apt-get upgrade with Leste
deb https://maedevu.maemo.org/leste chimaera-devel main contrib non-free n900 deb https://pkgmaster.devuan.org/merged chimaera main contrib non-free deb https://pkgmaster.devuan.org/merged chimaera-updates main contrib non-free deb https://pkgmaster.devuan.org/merged chimaera-security main contrib non-free deb https://maedevu.maemo.org/leste chimaera main contrib non-free n900 deb http://maedevu.maemo.org/leste ascii main deb http://maedevu.maemo.org/extras ascii main