My argument is that in my home PFS game, the GM allowed the druid animal companion advancement to add to the elemental. Here is why:
There is a contradiction in the paladin description even before you add archetypes.
"The second type of bond allows a paladin to gain the service of an unusually intelligent, strong, and loyal steed to serve her in her crusade against evil. This mount is usually a heavy horse (for a Medium paladin) or a pony (for a Small paladin), although more exotic mounts, such as a boar, camel, or dog are also suitable. This mount functions as a druid's animal companion, using the paladin's level as her effective druid level. Bonded mounts have an Intelligence of at least 6."
"Once per day, as a full-round action, a paladin may magically call her mount to her side. This ability is the equivalent of a spell of a level equal to one-third the paladin's level. The mount immediately appears adjacent to the paladin. A paladin can use this ability once per day at 5th level, and one additional time per day for every 4 levels thereafter, for a total of four times per day at 17th level."
The second sentence in the second paragraph contradicts the third sentence in the first paragraph. "This ability is the equivalent of the a spell equal to one-third the paladin's level." ---So when you get the ability at 5th lvl, you use it like a Monster Summoning 1 spell(example: 1/2 CR pony) and at 6th lvl, you use it like Monster Summoning 2 spell(example: 1 CR horse). Not to mention, you only get it for 1 round per level after you call it.
I(and my GM) really can't envision a holy knight riding into battle on such a weak creature. Especially at the level 5 when most antagonists will "one shot kill" this called creature. Neither can pathfinder so they ADD the druid animal companion stats to the "called creature."
So, the same goes for the Stonelord Archetype. They call the small earth elemental at 5th and finally medium at 8th. That is a long time to have a bonded CR 1 (+0 CR with celestial template). A very WEAK ally. Even at level 8 the medium elemental will be CR 3(still with the +0 CR for celestial).
I know Rules As Written vs Rules As Intended arguments don't matter in PFS. But the Druid Animal companion progression added to either normal paladin mount or stonelords elemental, just give the creature a fighting chance to last to the end of the 1 round/level duration in our power-gaming society.
I can't be the only person that has played this class. What have you other PFS players and PFS GM's ruled at your tables? I would like to get an official review.