Too many stones in one place

While he does say that he sees the souls and not the people, in the same movie Lady Sif tells the Collector (when delivering the Aether) that Asgard cannot keep the stone because it already has one in its possession. And that is the Tesseract. So this means that in no way MCU's Soul Gem can be Heimdall.

Unless, of course, nobody knows that it is in him, not even himself. Maybe Odin knows, though "“ but Odin didn't say anything about the Tesseract being an Infinity Stone prior to the first Avengers' revelation. So nobody knows about the location of the Soul Gem in the MCU.

So Heimdall couldn't have used its power when the Dark Elves attacked Asgard and Lady Sif had no idea that there are already 2 Infinity Stones in Asgard. This fits perfectly with what Kevin Feige said about Thor: Ragnarok, that it will be one of the most important flicks of MCU's Phase III.

Head on to the next page for the location of the Time Stone!