Are spot enhancements inferior to hide enhancements

This is substantially correct as a summary of the numbers-based rules, yes.

The inference that Bluff is a vital skill isn't correct though. The Hide skill is used to avoid detection in the first place. It is possible, but very hard, to hide after you've been detected... but the game acknowledges that someone very clever and skilled could pull off a "vanishing act" like that after they've already been spotted, so it provides some rules for it. But using Hide after being detected is not the normal use of the skill.

A ranger stalking through the woods to scout out a forward position uses Hide to avoid being detected at all. A thief infiltrating a keep in the dead of night uses Hide to avoid being noticed by the guards on duty while they slip in and out. If they need to cause a distraction in order to hide again, it's because they've already failed at their primary use of the Hide skill somehow, and in such emergency situations "re-hiding" will not always be the optimal response. (Fleeing is probably much better suited to most situations.) Buying up the Bluff skill is far from necessary, as there are other ways deal with such a contingency. Investing significant advancement resources into becoming especially good at vanishing from plain sight while observed is only one way of dealing with it; a flavourful one, it's true, but an expensive and very specialised way of developing a character.

With a cursory search, I found a couple items. The reference I found indicated Heal Kits can come in +1 to +10, not just +2. I also found: Magic Item Compendium - Healing Belt – Gain a +2 competence to Heal Checks and has 3 healing charges. There are also Psionic Shards that can be up to +10 to a skill ability that can be applied to Heal.

I noted that these also have instructions on what it takes to craft the items. The belt for example requires Craft Wonderous Item, Cure Moderate Wounds, 500GP, 40XP and 1 day to craft. How much would the benefits and cost scale up if Cure Critical Wounds was used instead?

Using templates from existing items or the rules for crafting you could generate any number of items that can benefit your player character. You could also apply an item that grants a feat, or craft one and apply the feat Skill Master and specify Heal, this would provide a +10 to Heal Check.

Skill Mastery is also a feat the play could gain that grants a +10 to the skill. If you are writing the Prestige class you could incorporate Skill Mastery Heal as a special gained at a given level. You could possibly create an Improved Skill Mastery Heal that stacks for a second +10 if you want the PRC to have healing that powerful.

Hope you find this helpful.