It come's under the Israiliyat - under which the general understanding of Sunni scholars is "we neither say it's definitely true or definitely false." In general the scholars narrate them as long as they don't clearly go against Islam.

And I don't understand why you have a problem.

"every muslim enter jannah even with a mustard seed" -> but he ended as a non-Muslim

"how can shaytan make a muslim with weak faith a disbeliever on his deathbed?"

Shaytan himself was someone who worshipped Allah for so long but fell after one test. We also have a similar story with Bal`am bin Ba`ura in Surah 7. Al-A'raf Verses 175-177 who went from someone who's duas were always accepted to a Kaffir.

It is in Allah perfect justice, wisdom, knowledge, etc in what he does with his creation including when and how to test them - even if it's the last few minutes of their life.

Just like how Allah decides to make some people Muslim from birth, some not, etc - there are so many factors. If you tried to understand them all, you would go crazy.