Please correct me if I'm wrong however is this the most effecient method of healing an injured paladin within a selected group of paladins?

1. Click the portrait of the injured paladin in the left pane to show which unit it is on the map
2. Click a different palaadin
3. Use hotkey to activate the healing spell
4. With the healing spell cursor click the injured paladin

If this isn't the most efficient way to heal then please tell me. It seems like a lot of micromanagement during the heat of battle.

In Wc2 heal is not very useful the spell costs 6 mana per hit point healed.
In the heat of battle if you really really want to get and use heal use in in choke points to keep alive units that are directly in the front and being attacked by selecting paladins in the back that are not fighting to heal the ones in the front.
As a rule of thumb prioritize units being attacked by more than one unit as their life will usually get lower faster.
Also better use paladins that have a decent amount of mana because healing 1 or 2 hit points on a paladin probably wont save him from a usual ogre punch anyways.

Thank you. There are videos online of people healing their groups of paladins during normal open-field battle and I thought it was required to beat the game. Learning that it's not very practical to do so is a weight off my shoulders.

Are their any other unit types whose spells are necessary to use during the chaos of battle to beat the game and to be decent at multiplayer? If so, what spells are they?

< a lot of micromanagement >
THIS.
although it have new (when game released, after Wc1 controls) KB and mouse controls it is still not the same as next games in RTS.