I would imagine that you could define a thing as a focus for his magic, and call that a 'holy symbol.' In mechanical terms it is the same, but in narrative terms it's not a symbol of his devotion to a deity, but rather a personal symbol of his devotion to...whatever he's devoted to. It could be anything.
It just has to be something. Tying his powers to an item is part of the balancing for the class, I believe.
As for carrying Dent around in battle...I don't think there's any hard rules for a tiger being ridden, but my broad sense is that there'd probably be some issues. They're not as strong as horses for one thing, nor as big, and bodywise (this has nothing to do with rules) a cat doesn't have the strength in their spine to really support a man in armor. That suppleness comes at a cost in that sense.
That doesn't mean Aridha can't wildshape into a warhorse and storm around...but it will take some IC discussion to work that out. It wouldn't occur to her naturally.
In general, I wouldn't recommend taking a feat in anticipation of the tactic happening regularly though.
One thing to beware of vis a vis Magic Initiate too is that the spells you pick don't automatically adjust themselves to suit you. For example, when a Bard takes a spell of another class with their class ability to do so, the spell becomes a Bard spell for that purpose...it changes its spellcasting attribute to Charisma. The Magic Initiate feat doesn't do that. If you take Druid spells, especially ones that allow a save, you'll want a good Wisdom to power them.
