Oh Why Not? They have shifted their best pass rusher to that spot, in an attempt by new DC Philip's to recreate his Demarcus Ware out of Mario Williams. Mario is amenable and has lost 15 pounds to fit. Plus they are desperate for some DB help. Darius Butler plus a fifth seems do able, or they give us Barwin and a fourth for Brandon Meriwether.
I'm not as quick to assume that Phillips would be eager to part with talented depth at OLB (Barwin) until he sees how the roster actually shakes out working in the new 3-4 (though dumber moves have been made by others in the past). In that he hasn't even seen these guys in a camp yet, I'm not even going to assume that Phillips doesn't plan on possibly playing a hybrid 3-4/4-3 the first year as players adjust to potential new roles, and I'm taking everything that's said about who
will be playing what position - even what Phillips himself has claimed - with a grain of salt.
Williams is 6'6"+ and has been playing at 290-295 on the LoS. Reports that he's lost weight to adapt to playing OLB are perhaps offset by other reports (including from Coach Kubiak) that Williams' off-season weight loss was due to nasal surgery to correct for obstructive sleep apnea. In any case, getting down 15 pounds, to 275-280, would stll leave him pretty hefty for an OLB.
To be clear, it's not that I don't think Barwin would be good with the Pats. I just don't think he'll be available.
As for Manny Lawson, I think he is a complete OLB, able to cover the pass and stop the run. While he has not produced double digit sacks, [yet], he has produced 4-5 a year, and that is young Vrable territory. Meanwhile the 49ers traded up to 8th to draft Aldon Smith as a replacement. I would guess that Manny could probably be obtained for a third and a fifth/sixth.
In his 5th season as a starter, Lawson is barely averaging, statistically, what both Vrabel and Ninkovich put up in their FIRST years starting at OLB, so it's not clear that Lawson would
necessarily be a significant improvement and a significant improvement is certainly what I'd want to see in exchange for a 3rd PLUS a 5th/6th.
Also, I'm not willing to presume that Aldon Smith is a replacement for Lawson. Smith might be a good size for a BB-style 3-4, HOWEVER, AFAIK, Harbaugh will be sticking with the 3-4/4-3 hybrid that the Niners have been playing for the past few years. In that case, Lawson, at a fairly nimble 240, would seem the better fit at OLB than Aldon Smith at 263 who is actually much closer in size to the Niners aging DE Justin Smith.
I would think bad-back Mathias Kiawanuka could be obtained even up for bad-back Nick Kaczur. Both were second round draft picks and they are position surplus to their present Team while providing a needed help to the other team. Nick was a starting tackle on multiple Super Bowl clubs.
AFAIK, (drated #32 overall in 2006, BTW) Kiwanuka is an unrestricted free agent. I think he'd be a good deal at that price, but I'm not sure I'd want to trade much for him, even your proposed Kaczur swap.
Kamerion Wembly was obtained by the Raiders for a mere third round draft pick from the Browns. Would they trade him for a Second which they gave the Pats this last draft? He is more valuable having finally produced a double-digit sack season, I would think, despite his other limitations.
Wimbley was a key front-seven player for the Raiders in 2010 - enough so that they gave him an $11.3M one-year contract which the Pats would have to assume in addition to giving up a 2nd rounder (at least that). I really don't see that deal being even close to ideal for either team.