Spear of Solaris Support Gems

I've been using spear of solaris quite a bit and have some feedback to give. Some of this feedback might be because I don't know enough about the back-end of this skill but I want to at least mention some observations I've made:

Spear of Solaris has a duration, AoE, and creates a ground effect. Here are my observations of some support gems that feel like they should work

Support Gems:
♦ Suffuse - I honestly can't tell if the AoE is increasing over time.
♦ Despoiler - Increased duration of ground surfaces created by a skill but I cannot socket this support gem into the skill. However, I can socket Suffuse which is a ground effect support gem. Why can I socket one but not the other? The base ground effect duration is 4 seconds so this should increase it to 6 seconds with the support gem.
♦ Persistence - Why doesn't this affect the duration of the spear? Spear lasts 4 seconds and this would push it to 5.6 seconds for 8 extra pulses.
♦ Expanse/Magnified Effect- I still can't tell if this is increasing the AoE. I added this with magnified effect and it still looks the same as if I had neither of these two support gems on.

Maybe it's not intended for this duration to increase but I just thought it would be a neat interaction to have and fantasy to focus on beaming down a giant laser beam with an extended duration.