It appears part of opengapps survive a system update (or installed apps assume it is there because it was at setup time, and freak out if it isn’t). Previously this had led to bootloops if opengapps wasn’t installed immediately after installing the FP openOS update package, as described here.
Why does this matter: I’d say there’s a fair chance that if you flash the old system without either installing opengapps or wiping your data or doing something smart that I don’t know about, you’ll still end up with a misbehaving phone. You might not run into that problem, but if you do, reflashing opengapps might solve it. If you do the latter, remember to download the latest version of opengapps for whichever android version you’re installing it (different versions for Android 5 and Android 6).
Chances are that your battery issues are related to the malfunctioning gapps installation.
Try the forum topics for the respective versions, IIRC there’s a request for a direct link for the manual install version in most of them. There is at least a direct link for 16.12, which you mentioned you were on before. You will need to use fastboot to install those images. No need for installing intermediate versions.