[RPG] Do desert heat hazards actually pose a threat to a Pathfinder party

Paladin as a tank / healer in combat is possible, but inefficient.

Building a Pathfinder character is all about tradeoffs. By mixing roles of frontliner and healing support, you can't really specialize at both effectively; you output less damage than a dedicated attacker, and restore less HP than a dedicated healer. Support casters often struggle in the front lines, because they're either busy attacking (i.e., not healing) or are taking too many hits.

Paladin is an ok choice, although their healing and support role is secondary to their melee power. Paladins are suited as a tank in combat, and a support/healer out of combat. However, they can still manage the combat medic role if planned and played strategically, and it sounds like you're set on pure paladin.

Usually, healing is best performed out of combat. Early on, a wand of Cure Light Wounds will be helpful. Eventually you'll want wands of higher-level paladin spells like Lesser Restoration.

A paladin's healing/support capacity improves at medium levels (5-10, usually). Combat healing in Pathfinder is suboptimal, because HP is typically lost faster than it can be regained. Therefore it's better to prevent incoming damage, by engaging and defeating enemies, than to spend your turns as a band-aid. There are exceptions (e.g. Heal) but paladins don't reach that level of spellcasting.

Combat tactics:

• Buff and heal sparingly, as needed. Your spells per day are very limited. Outside of melee, you should rely on wands and scrolls.

• Don't wait for the enemies to rush your party. Move into tank position as soon as possible. If you expect a difficult fight, maybe cast a buff spell for yourself and/or allies on round 1.

• Prioritize tanking over healing. When you're in melee and a nearby ally is getting low on health, the enemy is your immediate threat. Deal with the enemy first, then assist your allies.

• Encourage your allies to carry healing items (e.g. potions). If your allies can heal themselves via their own action economy, then you can spend your turns more effectively.

Useful feats:

• Power Attack. Useful for many tanks, especially with 2-handed weapons. Select your tank and frontliner feats early, since you can't cast at low levels anyway.

• Reward of Life. Make your Lay on Hands more effective, because Paladins have more daily uses of Lay on Hands than spell slots for healing spells.

• Combat Casting. Although 5-foot stepping away from your opponent is often the easiest way to cast spells, sometimes it's not enough (e.g., your enemy has natural reach) and you'll need to cast defensively. Concentration checks are tough for a Paladin because their caster level lags behind the primary casters, and so a +4 bonus will help.

• Selective Channeling. Tanky characters tend to be near enemies. This feat lets you channel positive energy to heal your allies without helping your opponents.

Archetypes:

I recommend none. Many of the paladin archetypes replace one or more of your Aura class features, which is a mediocre tradeoff. Because your Aura abilities are passive effects, exchanging one for a standard action ability hurts your action economy. Also, since you're going to level 20, your party will need reliable saving throws versus the onslaught of save-or-die effects; they benefit more from the Aura's bonuses.

• Rugged Faith appears like a significant upgrade, especially in combination with wild shape, which abandons the use of armor anyway. All natural weapons being favored weapons is a slight upgrade as well due to wild shape weapon dice being all over the place and therefore sometimes getting upgraded by sacred weapon. Not being able to wear metal armor is a slight downgrade outside of wild shape, but isn't practically more than -1 or -2 AC, even if you don't use ironwood or other similar solutions.
• Casting changes Druid list is somewhat worse than the cleric one, losing out on some powerful self-buffs and utility spells. But overall not a huge difference. Not being able to freely cast your highest level spells is likely less restrictive than it looks for combat due to always being able to pair combat wildshape forms with your best combat spells. Its a not-insignificant hit to out-of-combat utility. The spontaneous casting change feels mostly an upgrade. Cure spells were very unappealing to warpriests because as a 6th level caster they quickly fell behind on how effective their healing was. These control spells are better, especially with the new fervor.
• Blessings Not much of a negative. There's plenty of good blessings remaining.
• Fervor Losing the self-heal and self-buff is an enormous downgrade, as it was how the warpriest massively outperformed in martial combat vs. what their base chassis would imply. Being able to cast one-sided area control effects is very powerful, but by my estimation, isn't as good as the buffs, especially because your spell DCs are going to fall behind pretty quickly. Swift action wild shape is pretty good as a ribbon, but doesn't move the needle much on overall balance due to the long wildshape duration.
• Mystic Wild Shape So, wildshape is really really strong, and this archetype is a much more effective user of it than the druid would be. Between sacred weapon letting you apply stacking enhancement bonuses to your natural attacks, sacred armor letting you pump higher AC bonuses than a druid could, and higher base HP, I suspect it'll be in very powerful in melee. In comparison to a weapon-bound warpriest, you have various polymorph bonuses and access to forms with way more natural attacks at full attack bonus. You trade essentially nothing for what feels like a big upgrade to combat effectiveness vs. the base warpriest.

Overall the archetype seems like a solid upgrade over the base warpriest, but remains Tier 3

I think the real trade here is cleric-list self-buffs vs. wildshape. And that's a very hard comparison to make due to the high variability on both abilities. I suspect wildshape comes out on top, with powerful forms with many natural attacks making up for any bonuses you'd get from self-buffs.

Overall, I think the archetype makes for a beastly (pun intended) melee combatant, but may not hit as much on the warden flavor as you might want. The control spells are saddled with poor spell DC progression, meaning that they'll have huge punch at low levels where you will get to cast them as a swift action to disable foes while also full attacking, but will become more ignorable at higher levels.

A few unclear things:

• How does the fervor charm effect work? Does it allow a save? Should I be referring to a particular spell for how it changes the target's behavior toward me?
• How does sacred weapon's ability to grant an enhancement bonus to a weapon apply to natural weapons? I assume you have to pick an individual natural weapon once you've wildshaped.
• Does fervor's swift action wild shape count against your uses of wildshape per day?