Since we're talking about wheel tethering, you won't be compressing/loading the springs and truly eliminating all body motion. As such, I'd leave the tranny in neutral, to eliminate any potential tugging on your shift cables. It's my theory (totally pulled from my a$$) that the cars sometimes have cable adjustment issues brand-new due to Porsche ocean freighting and trucking the cars while in gear, and wheel-tethered. So take that for what it's worth (maybe not much
). But when you're tying to "sprung" points on the car, once you cinch it down, it's so isolated that you may as well leave it in gear, as the tie-downs have it isolated almost 100% in those cases. The e-brake on your 911 should be adequate "insurance". So yes, set the e-brake. You know, I never think about it because I've never trailered a 911. The fact that it is rear-engined, is a significant consideration. All my collectables are mostly front-engined front-biased Detroit iron. Rule of thumb is that you ALWAYS trailer facing front, with the engine over-to-ahead of the axles, with enough tongue-weight to keep the rear axle of the tow vehicle planted and secure. When you don't have adequate tongue-weight, and you've got a big chunk of weight hanging BEHIND the rear axles of the trailer, you wind up with a "tail wagging the dog" situation, that can wreck you. With a 911, I'm GUESSING that you would want the car facing rearward, engine ahead of the trailer axles bearing down on the tongue. I can't give you anything scientific to go on, as I always set load balance on my trailers by sight and instinct, how far the tow-vehicle's rear suspension is compressed, and the orientation of the trailer to the tow-vehicle, from a levelness standpoint. Suffice it to say, when you've got the fore-aft of the car's position correct, rolling down the road at 70mph should be a relaxed process, with little-to-zero trailer trying to tug you around, no wagging, and adjusting for semi truck wakes should be easy and uneventful.
There's gotta be a race-track-rat 911 owner on here somewhere that can authoritatively corroborate my guess that you'd want to back her onto the trailer...