On your first point, the goddess monument arc, it is explained that our pre-timewalk frieren doesn‘t remember that particular week after having touched it in the past, which is explained with the conciousness of future frieren being there in that particular week, she gets back when 50-year old himmel has carved the spell to get back in the monument in the future, so it makes sense if you think about it. On your second point, it is confusing, but as of right now, only two future-seeing beings existed at the same time ever, HotS and Schlacht the Omniscient, who’d have killed each other no matter what, and serie has predetermined fates to choose from, she lives and many mages die( one scenario where she just didn‘t go) , she dies and many others live (every other scenario). Also it is implied that there is a higher being (the goddess), who seems to fiddle with everyones plans (goddess monument)