Re: jahanum
Yes and no. Reciting istighfar is repenting for sins and seeking forgiveness of Allaah so whilst it doesn’t actually mean a guarantee from jahanum, it may be a means to protection from it. Depends if Allaah accepts it from us or not.
Then there is things such as sins involving other people, these require forgiveness from the people. That is why gheebat is to be avoided
Repentance is a major part of Islam which is these days not being given the appropriate attention
I’m not aware of any dua which can guarantee protection although there is duas requesting protection. We need to live a life of piety (obedience to Allaah) and be taken from this life as people upon whom there is reward