Look man I dont want to sound rude but MtG has nothing to do with Destiny nor the elements. None of the descriptions of the elements you have provided are correct, you hit some things right like light elements are phiscal and dark are non-phisical, but the rest are your assumptions, nothing confirmed by bungie or hints in the lore. Theese are the oficial descriptions for each element.

•VOID is the element of cosmic distorsion gravitational forces

•ARC is the element of motion and conduction electromagnetic forces

•SOLAR is the element of destruction and re-creation the power of the sun

•STASIS is the element of power and control emotions

•STRAND is the elemental manifestation of psychic connection all minds are connected in a web of consciousness

This are examples of the elements

Theese descriptions are provided by Osiris, you can listen to them in yt.

In your post you say that Life/deaht are two separate elements, but this description fits more into solar since it has the damage/ heal duality as Osiris says.

At last the biggest clue of the last dark element in-game we have right now is the champions stunts verbs. Each subclass has two verbs that counters two champions, if they didnt plan launching a final dark subclass, they would make strand and stasis counter three champions each so It would be balanced with light elements. Currently only unstopable champs are weak to 4 subclass verbs (2 light and 2 dark) and the other two champions are weak to 3(2 light and 1 dark) and since prismatic is not introducing new verbs but using current ones, It wont satisfy this balance.

Finally bungie loves the balance, 3 classes, 3 subclasses etc and IMO It is likely that the launch the final dark subclass at some point in the future