Acquiring Plant Allies Is Difficult without D&D 3.5 Materials
Getting plant allies as a cleric--even if the cleric has the plant Domain--is just not a well supported option in Pathfinder. Summoning them appears impossible, and even making Diplomacy skill checks versus low-Intelligence plants (wild plants?) requires the wild empathy ability then the feat Greater Wild Empathy, and unlike so many things in Pathfinder (like the once class-feature-only ability to use the Disable Device skill to disable magical traps that can now be gotten with a trait) wild empathy is--so far--only ever offered as a class feature, requiring multiclassing into druid or ranger or whatever.
There are only a handful of options, and none of them are what you want, but maybe you can make do.
The Staff of Because Talking Plants Are Awesome
Aura: Transmutation and abjuration; CL: 9th; Slot: None; Price: 12,070 gp; Weight: 5 lbs.
Description: This hollow steel staff depicts clerics in metal armor carrying light steel shields and wielding morningstars beating up sobbing hippies. When used as a weapon, it makes a deeply satisfying hollow clang that seems louder when the victim is a druid. The staff allows use of the following spells:
- awaken (5th-level Drd spell) (10 charges)
- shield of faith (1st-level Clr spell) (10 charges)
Construction Requirements: Craft Staff, awaken, shield of faith. Cost: 6,035 gp (Math: ((400 gp x 5 for the spell awaken x 9 for the caster level) + (2,000 gp for material components x 50 to charge the staff) / 10 for 10 charges per use) + ((300 gp x 1 for the spell shield of faith x 9 for the caster level) / 10 for 10 charges per use)).
Note: A cleric can recharge the staff of because talking plants are awesome because of the presence of the shield of faith spell in the staff but must make Use Magic Device skill checks to employ the awaken function. The awaken spell has a 24-hour casting time anyway, so whatever. This staff permits a cleric who has access to 5th-level spells and is willing to commit the resources to being able to use it and recharge it to awaken a plant once every 10 days. You weren't going to use it to cast shield of faith anyway.
The Crown of Bow before Your New Master, Plant Dudes
Aura: Transmutation; CL: 9th; Slot: Head; Price: 36,200 gp; Weight 1 lb.
Description: Once per day on command, this light steel crown depicting moping hermits being evicted from a forest by a valiant metal-armored cleric allows the wearer to spend 24 hours to use the magic of the awaken spell.
Construction Requirements: Craft Wondrous Item, awaken; Cost: 18,100 gp (Math: ((1,800 x 5 for the spell awaken x 9 for the caster level) + (2,000 gp for material components x 50 for an uncharged item)) / 5 for 1 use per day).
Note: The spend 24 hours in the description is not a new requirement but a reminder of the awaken spell's casting time. This has the advantage of being usable by anyone without needing the Use Magic Device skill.
You are right that the key word here is "communicate," but unfortunately, because that word has no specially-chosen meaning in the RAW,
The answer varies depending on the creature summoned, and may not be covered clearly by RAW alone.
However, here are cases clearly covered by the RAW:
- Obviously, summoned creatures obey commands given in any language they can understand (listed in the Languages section of their stat blocks).
- Summoned creatures with Intelligence 2 or 1 can be commanded by using Handle Animal normally. The check is DC 25 (30 for non-animals), and requires a full-round action. (Because there is no evidence that summoned creatures are trained as an attack dog would be, you must use the "push" option from the skill.)
- Numerous special abilities and spells may be helpful, such as Speak with Animals.
The GM's intepretation of the RAW may permit other options:
- Commands may possibly be obeyed via Diplomacy, but only if the creature can "understand" you, has intelligence higher than 3, and "generally" only outside of combat. "Understand" and "generally" are also not clearly defined, so good luck.
- As a last resort, a Charisma (scroll to the bottom) ability check may be permitted when nothing else applies; discussions of RAW often miss the basics, but the basics are there for a reason: "You apply your character's Charisma modifier to . . . Checks that represent attempts to influence others."
I don't mention the Charisma check as a joke: Simple commands like "Attack that guy" can be implied well enough with two or three words and some pointing. Anybody can memorize two or three key phrases in several different languages. But that kind of thing is subject to GM interpretation.
Looks like in Pathfinder, only the caster summoning the monster can dismiss it. The one who casts Control Summon Monster can only dismiss their control over it.
(Disclaimer: This will only look at the two summoning spells listed in the question. For all other summon spells, please look at the description for that particular spell).
So for the two summon spells of Summon Monster and Summon Nature's Ally, you're looking for the subtle (D) in the duration. If that D is there, then it can be dismissed at will by the caster. The duration for both of these spells are "1 Round / Level". So only the one who casted the summon spell can dismiss them, otherwise they last for the full duration, determined by the level of the caster.
Meanwhile, with Control Summoned Monster, there is no mention of being able to dismiss the monster. In this spell, it does describe that the original summoner can perform an Arcana check to try to win back control. If the check succeeds, then the Control Summoned Monster spell is removed, and would have to be recast to try again. In the meantime, that spell also has a (D) in its duration description, so that spell can be dismissed. But that spell only pertains to the control of a summoned creature.
Also, they would have to worry about controlling a stronger spellcaster's summoned creature. Because the duration of their Control Summoned Monster spell may not last as long as the Summoned Monster spell, and may wear off before the monster disappears on its own!
Lastly, since the general rule of dismissing the spell is there in the duration for the summoning spell, the caster has the ability to dismiss it at any time. This also means even when that summon is controlled by another caster, because the text of the Control spell does not specify that it has the ability to prevent this.
So good news! The thief spellcaster cannot just steal your summoned monsters only to dismiss them. They would need something like a Dispel Magic spell to do something like that. Sure you can dismiss it if someone does steal it, but give an Arcana check or two a try before you do.