I know ranger/clerics can memorize and cast druid spells. Does this apply to HLA's as well? I know when they get to select HLA's that they can chose from ranger and cleric HLA's, but will they be able to chose from the druid HLA's as well, specifically greater elemental summoning?

As far I know, they can't select druid high level abilities. But aren't they good enough without that?

From what I know, rangers are much like treehuggers, and that why they have the ability to cast druid spell.
I believe ranger/cleric can cast druid spell just because he is partly ranger. So the druid's HLAs are not included, as he is not druid.

Then it's kinda strange that he can cast level 4+ spells, since a vanilla ranger can't...

There are certain spells which both druid and cleric can cast. So I believe that those lv 4+ spells are there because of the cleric's part. In short, I think ranger can cast druid spells but not all as he is more skilled with weapons and cleric can cast druid spells but not all as he only concentrate on the anti-unholy, protective and healing type.

Nope.

The ranger gets druid spells and the cleric adds the extra spells.

Cleric never gets druid spells. Honestly, I think the ranger/cleric getting the level 5+ druid spells is an engine limitation.