i'm gonna throw a theory out on the clipping (and the antenna). feel free to shoot this down if you'd like, i'm open to ideas.
so, i started out the process of trying to test the antenna on the roof. i didn't want to VHB it down just yet, in case i decide to leave it over on the corner...so i just put some duck tape at the base of the antenna to keep it down as much as i could, and then went for a short drive around the neighborhood (including under the freeway overpass i mentioned earlier) to see how it performed. the answer is: not bad, but not great. not sure it was good enough to justify all the hoops you'd have to jump through to hide the wires from up there vs just putting it on the corner where it's easy to just run it under the trunk weatherstripping and up to the tuner from there.
granted, the antenna wasn't stuck down very well...so that may have had an effect on the test. i'm gonna see if i can come up with a better way to keep the antenna down and try this again. the interesting thing though is that the antenna flopping around on the roof gave me the theory on the clipping: the signal kept cutting out, because the antenna was flipped straight up in the air. it would occasionally come back on, depending what direction i turned...but mostly stayed off (unless i turned into a direction where the antenna could grab the signal). also, even sticking up in the air - when i kept my speed under 25mph or so...the signal stayed on.
it also then hit me that since i moved the antenna outside to the corner, i haven't noticed the clipping nearly as much. i noticed that the buffer still doesn't seem to be 7 seconds while it's there, but the clipping issue has been reduced. i won't say it has been completely gone, but it has been significantly better.
maybe, depending where the antenna is (and potentially related to the potential shadow we talked about from the C-pillars and the roof pillars when the antenna is inside on the rear deck), if the antenna can't the signal into the tuner fast enough, it starts clipping like we've been talking about. that would explain why i heard it so much more noticeably when backing into my garage, right before the signal cuts...it was being shaded and then completely dropping when it got into the shadow of my house.
one thing is for sure at this point: i'm not moving the antenna back inside. it'll either stay on the corner opposite the charge port, or i'll find a way to get it up on the roof...but it does seem to make a difference vs inside. small sample size so far, but i'm going to leave it outside for an extended period now one way or another and get us more data.