Plastic straws cause several issues: it will be difficult to push the Lightning connector through, they won’t shrink as easily or uniformly compared to shrink tubing, and they’re rigid so there will be a hard edge at the end, likely creating a strain point where the cable will fray. Heat shrink tubing is readily available at hardware stores and it’s not expensive.
If you followed my earlier recommendation, this is still connected to the logic board at the other end. It does need to be removed from the speaker as shown here.
The shorter cable is disconnected at both ends and will come free. If you followed my earlier recommendation, the longer cable is still connected at the logic board end and so needs to be separated from the speaker but left in place in the phone.
This step isn't necessary. You can leave this cable in place.
From here you can skip ahead to step 50.
It is possible to perform this repair without separating the display from the back, and without removing the logic board (which means you need not remove the camera, cellular antenna cabling, audio cabling, and other components). I carefully completed steps 1-15, 38-39, 50-52, and 54-60. It's also not necessary to disconnect the WiFi diversity cable from the logic board . This saves a lot of time and greatly reduces the risk of damaging delicate parts.
The copper sticker is some kind of ground connection, and is therefore probably very important given the frequencies present inside the phone, and that the phone is itself a radio transmitter and receiver. I too followed the instructions and removed it but I was super careful and didn't tear it, and was able to reattach it. I used the spudger to smooth it and ensure that it makes good contact with the shield. It didn't even occur to me to skip the removal step! But I agree with that advice.