Risks of removing the stock browser on LineageOS
I ended up going ahead and just removed the Jelly browser – initially manually by using TWRP's file manager and later – since it came back when I updated the system afterwards (duh) – using a handy flashable zip that I got from LOSdiet. So far, so good. Everything seems to work fine.

As for WebView, the Android System WebView entry in the developer settings is still there, so I suppose WebView is not tied to the Jelly.apk. Indeed, the LOSdiet site github.io page as well as the old CyanogenMod "barebones" wiki page indicate that the default (pre-installed) WebView is a separate .apk.