X-Men #12-13 are collected in
Bishop's Crossing.
And yes, the first few issues do credit Portacio along with Lobdell as co-plotter. I agree about Portacio's art; at the time it looked like a super sloppy imitation of what Lee was doing in the sister book, but I came to appreciate the stories. Great stuff if you're a Colossus or Archangel fan.
X-Men #14-16 are collected in
X-Cutioner's Song . And yes, #17-24 are in both the
Skinning of Souls TPB and the
Shattershot OHC. That last one also gives you some annuals and the
Survival Guide to the Mansion one-shot.
As they've slowly filled in the gaps, I've been getting rid of my single issues for space reasons, and it's been like pulling teeth! I have to argue with myself why I don't need multiple copies of #20 just because I love the cover and Revanche is the sh!t... *eyeroll*