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.
No, they can't be enchanted, because they aren't masterwork and can never be, and all magic weapons must be crafted with a masterwork base.
Creating Magic Weapons
To create a magic weapon, a character needs a heat source and some iron, wood, or leatherworking tools. She also needs a supply of materials, the most obvious being the weapon or the pieces of the weapon to be assembled. Only a masterwork weapon can become a magic weapon, and the masterwork cost is added to the total cost to determine final market value.
It is quite plain that they are not masterwork by default, given the lack of +1 to attack rolls. So let's suppose we want to upgrade them. But it is impossible for an existing unarmed strike to be made masterwork, per this clause in Weapons:
You can’t add the masterwork quality to a weapon after it is created; it must be crafted as a masterwork weapon (see the Craft skill).
All right, let's try making a new one that's masterwork. This fails because unarmed strikes are not manufactured weapons, and you can't manufacture a weapon with the Craft skill if it's specifically not manufactured at all.
Glossary — manufactured weapons:
This category also includes […] in essence, any weapon that is not intrinsic to the creature.
Monk's class features do not help with this, since Craft is not an effect and neither enhances nor improves anything:
A monk’s unarmed strike is treated both as a manufactured weapon and a natural weapon for the purpose of spells and effects that enhance or improve either manufactured weapons or natural weapons.
How about buying one? You might actually be able to buy an ordinary unarmed strike, since they're listed in the Equipment section in the Weapons table, and you needn't pay anything for it*. Unfortunately, per the quote on MW weapons, a weapon must be crafted as masterwork to be MW, and per the preceding this is not possible even for NPCs. There might be an exception, as there seems to be for magic oils, which you can buy in shops but not craft, except that nowhere is "MW unarmed strike" listed for sale.
Magic fang and company do not make a weapon into a "magic weapon"; rather, they give a weapon a +1 enhancement bonus (and the ability to bypass DR /magic because of that), which is not technically the same thing. And while all magic weapons are masterwork by definition as well as by prerequisite, magic fang and friends do not have to be interpreted as giving a weapon retroactive masterwork status, for essentially the same reason, which is fortunate as otherwise the rules would create some bizarre and abusive situations†.
Magic Fang
Magic fang gives one natural weapon of the subject a +1 enhancement bonus on attack and damage rolls.
Magic Weapon
Magic weapon gives a weapon a +1 enhancement bonus on attack and damage rolls. (An enhancement bonus does not stack with a masterwork weapon’s +1 bonus on attack rolls.)
You can’t cast this spell on a natural weapon, such as an unarmed strike (instead, see magic fang). A monk’s unarmed strike is considered a weapon, and thus it can be enhanced by this spell.
Magic Weapons
All magic weapons are also masterwork weapons, but their masterwork bonus on attack rolls does not stack with their enhancement bonus on attack rolls.
Special Abilities
Some monsters are vulnerable to magic weapons. Any weapon with at least a +1 magical enhancement bonus on attack and damage rolls overcomes the damage reduction of these monsters.
Note the phrasing of the last quote; they are described as vulnerable to magic weapons, but that's not the rules text, just the plain description, since vulnerability is not defined (except in the incompatible context of energy vulnerability; it should be clear that DR 5/magic does not mean that a +1 longsword does 50% extra damage as well as overcoming DR). The actual rules say that any weapon with a +1 magical enhancement bonus (whether or not the weapon is, per se, a "magic weapon" itself otherwise) overcomes that DR.
(emphasis added to quotes)
* This is, to my mind, patently absurd, but hey, RAW. Monks won't be proficient with those unarmed strikes, but hey, RAW.
† For example, casting magic weapon to increase the price of a club from 0gp to 300gp (permanently? who knows!) and selling it. Or casting magic weapon on any plain weapon to make it masterwork for future crafting. There is a Pathfinder spell for this purpose. It does nothing else‡, is a level higher, costs as much as crafting a MW weapon in the first place, and takes an hour to cast. Allowing magic weapon to do this for free in a standard action is ridiculous.
‡ It has this excellent provision: "If the target object has no masterwork equivalent, the spell has no effect." Doesn't work on unarmed strikes, as they appear to have no MW equivalent and are not objects.
Best Answer
When the Dungeon Master's Guide says, "A single weapon cannot have a modified bonus (enhancement bonus plus special ability bonus equivalents) higher than +10" (221), the Guide's discussing a magic weapon that's created with a modified bonus higher than +10, not limiting an already magic weapon's capacity to be further temporarily magicked beyond an effective +10 bonus.
The Magic Item Compendium clarifies that
(Emphasis mine.) This forces the reader to the Epic Level Handbook, which says
So a magic weapon can even be created to have an astronomical effective bonus if a creator is willing to pay an astronomical cost, and, by extension, temporary magics pretty much don't count at all for any of this because temporary magics don't increase a weapon's gp value. (Disregarding the possibility of a con artist duping a mark into thinking a temporarily magicked weapon is an epic weapon, of course—that's not a game mechanic but a plot.)
Admittedly, the Dungeon Master's Guide should've been more up front that it's all about creating weapons not casting spells on weapons rather than making the reader bounce from book to book to confirm.
For example, this means a +1 briliant energy vorpal longsword (hence possessing a modified bonus of +10) can benefit from a Wizard 20 casting on the weapon the 3rd-level Sor/Wiz spell greater magic weapon [trans] (PH 251-2) so as to grant the weapon instead a +5 magical enhancement bonus, and the weapon can also benefit (although not much) from a level 20 cleric casting on it the 4th-level Clr spell undead bane weapon [trans] (Spell Compendium 226), even though, combined, these spells would make the +1 briliant energy vorpal longsword effectively a +5 undeadbane briliant energy vorpal longsword (a weapon with an effective bonus of +15).
Looking at things from another angle
One of the game's centerpieces is that a specific rule can override a general rule. This allows the rules of the specific spell greater magic weapon to override the broader, general rules about magic weapons presented in the Dungeon Master's Guide, essentially allowing the spell greater magic weapon to function as per the Player's Handbook on weapons that already possess a magical enhancement bonus, magic weapon special abilities, or both so long as one keeps in mind the rules for stacking bonuses.