How strong is the adhesive on the iFixit replacement battery, compared to the Apple OEM adhesive? Are we looking at a similar painful extraction process if/when we pull the replacement battery?

For that matter, is an Apple-esque strong adhesive actually necessary in this application? Double-sided tape is usually weaker; it might be a little thinner than the adhesive layer, and certainly far more user serviceable. Would tape be sufficient to hold the battery in place? Once you've got the bottom case screwed back on, the battery is immobilized anyway.

The epoxy cementing the rubber pad to the aluminum base is a lot stronger than the rubber itself. The rubber is likely to tear as you remove it; I had to use a putty knife to clear the screws. It's worth hanging onto the branding near the Ethernet ports, for future identification (model number, serial number, Ethernet ID, AirPort ID); you can fashion a replacement pad out of neoprene rubber.

In both the online and the PDF version, Steps 17-18 and Steps 29-30 are identical. At which stage should you actually remove the display screws?

I use a screw ID technique similar to griffn's; I print out the PDF version of the guide, then Scotch-tape each screw type to the guide at the step where the screw was removed. The screws are then distinguished by size and type at the point where they need to be reinstalled.

I use a similar technique: I print out the iFixit manual for the job, and Scotch-tape down the screws/brackets/cables I remove at each step next to the component descriptions. That way, when I'm reassembling, the bits are taped right next to the photo of where they came from.

When reinstalling the battery connector/cable, placement of the connector is critical. If the connector isn't positioned accurately, contact with the battery terminals won't be completed. The battery connector cover will help you align the connector, but there will still be a little leeway as you screw the cover down. Position the battery first, to adjust the connector's position; then remove the battery and tighten the cover screws down. You may even want to try plugging the computer into the AC charger before completing the cover attachment, to ensure that battery is charging and the connector is correctly positioned.