TLDR: If you want a Mk12, build a Mk12. If you want a lightweight gun, build a lightweight gun. Just don't expect the 2 to overlap without taking some exceptions to the original design.
The 2" difference in barrel length won't make much difference in terms of weight or performance. The heart of the Mk12 is the barrel, so I don't really think you can change to a light profile and still consider it a Mk12. Whether you go with a 16" or an 18" barrel, the difference is only an ounce or two - which isn't really going to change the way the gun feels. If you go with a light profile barrel, then you might as well just build a 3-gun rifle and forget any pretense of building a Mk12.
While I've always wanted a Mk12 Mod 0 style rifle, I thought that technology had advanced in the following decades and that I'd rather have a lightweight rifle. So I built around an 18" LaRue PredtAR barrel and Geissele Mk4 rail to have a middle-weight, do-everything gun. While I'm really happy with how it turned out, nothing really compares to a Mk12, and I'm going through the process of building a Mk12 anyway. I learned a lot from that build, so I don't regret it, but if I built the Mk12 first, I probably would have skipped the 18" lightweight gun for a more handy 14.5" Recce/3-Gun style build (that I'm also working on).
However, if you don't care about having a "clone-correct" gun and want to cut some weight from the Mk12 Mod 0, I think the heavy SWAN sleeve is obsolete, as it just adds redundant weight to allow for 2-piece rings to be used. Now that modern optic mounts are available in cantilevered designs, they get you to the same goal (precision oriented optics mounted on the AR platform) without the added cost & weight.
Living in California, my Mk12 is going to be some sort of abomination regardless, so I figure I'll just build it the way I want and be happy with it. The "purists" may balk at the franken-gun construction, but I'm not building mine for the purists -- I'm building it for me. My Mk12 (Mod CA?) is going to utilize the Gen3 Delta rail, skip the SWAN sleeve, have a featureless grip, use a carbon fiber stock (Lancer), and obviously skip the suppressor since my state won't allow them. If I ever move to a free state, a suppressor will be at the top of my to do list, and at that point I may consider more "clone correct" parts. But as long as the barrel, handguard & brake/collar are there I'll be happy enough. Attached is a mock-up that shows the general idea of what I'm planning (with some substitutions for parts not on gunstruction).
. It's not a whole lot lighter than my 16" .308. How much more performance do you get out of a Mod 0 than the Mod H? Do 2" of barrel make that of a difference? When I get my stamp back on my AEM5, the Mod 0 is gonna pretty damn long as well.
