Heroes 5.5 WarGame Edition - Tactical Edition.

Yes, unfortunately, the Rain spell isn't tailored to the native Elven Summoning skill.

I think it's possible to adjust it – reach out to an expert in executable files to compare how the 3.0 and 3.1 executables differ in terms of Summoning spells and patch it.
By the way, in the 2.1 executable, you could apply the General Overseer buff, after which the ability was lost. The Overseer buff exists on units, but there's no effect. Now, only the General Support skill remains (it's quite powerful – 5 stacks of Centaurs, each with Support, and the hero attacks with a stun. It's quite the control...). And there's more to explore in that direction...

Native – because, with the exception of Enchanted Arrows, Summoning is simply perfectly suited for Elves, both in terms of lore and gameplay (unlike the Necromancer), due to their high defense.

I completely agree. Light – Light + Summoning, Dark – Chaos + Darkness. That's how it should be.

Tier 1 spells, including Chaos and Summoning, will cost only 1-2 mana.

What about the Shaman-Mages? Low-level spells are cheap, high-level and mass spells are expensive. There are Shamans that have both mass/high-level and low-level spells. Give them a lot of mana so they can cast a mass spell at least once? But with 1-2 mana, they'll be overwhelmed by low-level spells.

Maybe just reduce the mana from Knowledge? Instead of 10, give less? This is quite adjustable.