Natural Ambition is an Ancestry Feat that specifically says you gain a 1st-level class feat for your class. The multi-class rules say you can spend your class feats on archetype feats. Since Natural Ambition is not a class feat, you cannot use it to spend on archetype feats.
I mean, besides the obvious interpretation that Natural Ambition allows you to choose FEAT .
But hey, feel free to allow it in your homebrewed games, I doubt your interpretation will be allowed in 'official' games.
DC - Notes
5 - This is what you always start with
+0 - Requires Craft Wonderous, you cannot skip this requirement
+0 - Requires Keen Edge, which you say you have as a spell and can cast it
+5 - CL5 as listed, note that it can be reduced to minimum casting level, but it already is at min.
+5 - if you want to craft in half the time (16 days is long, so half it)
Total DC is 15 (or 10 if you want to spend 16 days crafting, but you probably don't and you cannot fail anyways). As Dasrak said, take a 10 on your skill check: 10+11=21. Congratulations, after 8 days (spending 8 hours straight) and 8,000 gold (half of the market price) you have a Scabbard of Keen Edges.
Note that unless I am reading it wrong, the time it takes to craft a magic item is 1 day per 1,000 gp in base (market) cost.
If you don't have time to craft an item, you can still work at it while adventuring. All work must be done in 4 hour blocks to count as actual 4 hours of crafting. If you cannot do so, then you can still spend 4 hours toiling and still get 2 hours of crafting out of it.
I suppose for PFS or the like it would work. I don't play that, and the campaigns I play in don't have regular retraining (in this case, we are going through an adventure path and we only get down time after we complete a book). Essentially you are trading out Intensify for Reach. Since we get Reach Metamagic as an arcana choice at 9th, I am not taking it early.
Not taking rapid shot that early as you already have it with spell combat, and more minuses doesn't help the early levels.