How can I tell a savegame is not safe for further progression?

Is it possible a follow up save is smaller in size than the one before?
It fails too often loading, crashing the game?

Experiencing lots of strange stuff, runewright breaks my witcher gear, enchantments disappear, single quest tasks are not recognized correctly as "done" , people were scared of me in velen for no obvious reason, newly i can consume multiple edibles at once stacking, giving godlike regen abilities. Have low health effects visible on screen being at full health. - the things do not appearat once but step by step ... :(

But now : how to tell what is the last fully working savegame ?

First, when something like these obscure, seemingly random occurences happen, the very first thing to suspect, and remove, are your mods, especially any mod/s that use/s scripts or code injection (eg. console enabler). You cannot troubleshoot such issues with mods installed.

uninstalled all mods and the debug console, the bugs still remained - had to delete hours of progression, all the saves where i was able to consume an unlimited amount of edibles. it got persistant to the savegames. all the way back to the quest task that didnt complete (was nothing important but optional), had to talk to the priestess about morkvarg - damn this game became really fragile - wish there was any kind of error correction, so bugs dont become persistent.

Of course it does. If you want the game to properly track everything that goes on, it must be persistent. This is the risk you take when using user made mods.

how you know its a mod fault?

Go and read my original reply again. I said, and I quote: "the very first thing to suspect, and remove, are your mods". With the ridiculously long catalog of issues you quoted, there can be only two things wrong: mods or corrupt installation... or both.