Re: ONE
salam alaikum
heaven and hell are a matter for Allah SWT to decide (ultimately)
however we have been given the Book of Allah which has banned sects, so yeah only one group/way/type is true muslim. as for recognition of who is to be part of this group the Quran and sunnah confirm that it is those upon what the Prophet SAW and his companions RA were on. so the standard is high, and that was the only group approved by Allah SWT
various groups/cults claiming to be a sect of islam have no real basis for the claim, although being a sect of islam could arguably be a proof of their disbelief initself, the scholars have not had to entertain the idea since the disbelief of these groups has been proven on more basic matters.
those monotheists judged by Allah Most Merciful to have some (little) iman, will eventually be allowed into heaven having served a period of punishment. however since we as individuals cant judge this situation we all fear our own positions.
all imo, corrections welcome