I think i have an idea how it could work with ammo type,
3 ammo or 2 if too op
1s cast time
internal cooldown 40s
if ammo count = 3, summoned elemental has 40s duration
if ammo count = 2, summoned elemental has 80s duration
if ammo count = 1, summoned elemental has 120s duration
so every 40s a elemental dies, like before.
If one or more dies "to too much damage = before 40s" and you summon with ammo 1 the next elemental has 120s like before so you need to slowly build up to 3 like before
Tell me if i am wrong but that should be mostly like the old behaviour except that you can cast more at the start.
to fix the mount question -> refill the ammo when mounted
PVE Only.
I am scared that it could lead to strange things in wvw or pvp.