[RPG] What caster level do you need to craft a Ring of Protection +5

It's Caster Level 8

Your second interpretation is correct. Here's the rule itself:

Creating a magic weapon has a special prerequisite: The creator’s caster level must be at least three times the enhancement bonus of the weapon. If an item has both an enhancement bonus and a special ability the higher of the two caster level requirements must be met.

Then from the Magic Weapons page:

In addition to an enhancement bonus, weapons may have special abilities. Special abilities count as additional bonuses for determining the market value of the item, but do not modify attack or damage bonuses (except where specifically noted). A single weapon cannot have a modified bonus (enhancement bonus plus special ability bonus equivalents) higher than +10. A weapon with a special ability must have at least a +1 enhancement bonus.

From here, it's pretty clear that enhancement bonus and special abilities are distinct things. You combine them for the weapon cost (as the rule stats), but not for the crafting caster level requirement as each type has its own requirement. You just have to be able to meet all the requirements.

Thus, you need CL 6 (3x2) to make a +2 enhancement bonus, and CL 8 (and have access to the spells Chill Metal or Ice Storm) for Frost.

This also means all the weapons on the chart can be created without being epic level, as if you needed to be CL 9 to make your +2 Frost Weapon, it'd be impossible to do any combination that is >= +7 without epic levels. (You can't make a +7 sword without epic levels, but you can make a +5 Keen Frost one, which has the cost of a +7 equivalent weapon.)

It Could Be Caster Level 6

Depending on how you want to interpret the rules around prerequisites, you could create the weapon at CL 6 as a Druid (who get access to Chill Metal at level 3). The reason for that is these rule passages. From the SRD:

While item creation costs are handled in detail below, note that normally the two primary factors are the caster level of the creator and the level of the spell or spells put into the item. A creator can create an item at a lower caster level than her own, but never lower than the minimum level needed to cast the needed spell.

And from DMG p. 215:

For other items [besides potions, scrolls, and wands] the caster level is determined by the creator. The minimum caster level is that which is needed to meet the prerequisites given.

Depending on how you read this, the prerequisite could only be access to the spell itself, rather than the CL listed. In that case a level 6 Druid could create the item (as they can do both +2 and have access to a required spell). A Sorceror would need to be level 8, as that's when they get access to Ice Storm.

I'm not 100% convinced this is how they intended it to work, but it makes an item entry like Universal Solvent make a lot more sense. It has a listed CL of 20, which for such a common an inexpensive item just seems really wonky if only the greatest spellcasters on the planet are able to create it (particularly since the required spell is available to Wizards at level 9).

If you use this interpretation, the CL listed on the item entries is a guideline, or what you'd expect a normal item of it's type to be if you find one in the world.

Yes, they stack like the rule you cite says, if the specific class says they do. This is an example of exception-based design - the general rule is "only the one class counts" but then the specific rules in the other classes can say "but I do."

Not all prestige classes are alike so you should carefully read their Spellcasting benefit. In this case, Arcane Trickster says "I stack."

But classes that do not say that phrase, like Wizard and Sorcerer, do not stack. In your second example, Loremaster's spellcasting is worded like Arcane Trickster (as if she had gained a level in a spellcasting class she belonged to...), and you would need to decide whether you want it to stack onto your 5th level wizard spellcasting or your completely separate 3rd level sorcerer spellcasting. Assuming the former, you'd cast your spells from wizard slots at level 8 and your ones from your sorcerer slots at level 3.

It's kinda obvious why this is, the core idea behind prestige class design is "continue what you were doing, down a slightly weirder path" and therefore completely penalizing a spellcaster by e.g. halving their spellcaster abilities would be undesirable (even at that, many don't add a spellcasting level at every level and have one or two "penalty" levels.