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.
Best Answer
It's a neat idea... but it probably wouldn't, and shouldn't, work, for a couple of reasons:
1. That's just not how magic item creation works in D&D.
As the rules quoted by Red Wullf say, you have to be a spellcaster to create a magic item in D&D. So unless your group is willing to introduce their own custom house rules for magic item creation, it just won't work. Your fighter can scribble anything they want on their sword, but at the end, without a spellcaster to help them, they won't have a magic sword, but just an ordinary sword with scribbles on it.
2. If it was that easy, everybody would do it.
Even if I were to introduce a house rule to allow enchanting items like this, I wouldn't let simply copying the runes on another item accomplish anything but the most minor enchantments.
Why not? Well, if you could enchant items like this, so could anyone else with a reasonably steady hand. If a fighter can carve runes on their weapon to enchant it, so could the blacksmith that forged it in the first place. So if enchanting items was this simple, everybody should be walking around with runes on their weapons (and probably everything else they own).
Conversely, if you want powerful enchantments to remain rare (and I presume you do; it would just be silly to have every random NPC walking around with a Flaming Ethereal Armor-Piercing Impervious Returning Holy Stunning Keen Extensible Soul-Sucking +11 sword of Triple Damage vs. Everything), there has to be some reason why not just anybody can create them by copying another enchantment. The two obvious explanations are that either:
it's difficult to copy advanced enchantments — so difficult that only people with extensive practice (i.e. high level in a suitable class) can hope to accomplish it reliably, or
it takes something more than just the right runes to enchant an item — perhaps the runes only serve to bind whatever being or force is powering the enchantment to the item, but do nothing else on their own.
Thus, sure, I'd be happy to house-rule that something like, say, a minor enchantment against rust could be applied to a weapon just by copying the runes off another weapon; this would then be a standard feature of any weapons of decent quality. Or I could easily see anyone with a bit of skill being able to inscribe the correct runes on a clay amulet to protect its wearer from evil spirits — you know, those ubiquitous evil spirits that will surely get you if you go out without a protective charm, which is why everybody has at least half a dozen such trinkets on them at all times. In short, the kinds of things that make for neat setting detail, but which don't really have a massive effect on game balance.
But if you just tried to copy the runes on a powerful magic sword onto a lesser blade, without any special understanding of what you were doing, then the only reasonable outcomes I can see would be either that they just wouldn't work (because you did them wrong, or because some other essential component is missing), or possibly that they would work, but not quite the way you intended... probably leaving you with a cursed weapon, and an object lesson in why mere laypeople should not dabble in magic. ;-)