You went for a very complex build buddy
1. Drivers are as good as you implement them. Good tuning, acoustical design, and crossover are huge factors.
2. It's hard to tell, as you're using a custom shell with no 3D-printed spaces for drivers. Generally speaking, low-frequency drivers can be used with longer tubes, while mid and high frequencies require tube length adjustments to set peaks at the required frequencies.
3. As mentioned above. The placement looks clean from the top, but it should be closely related to the acoustical design of your sound bores. From my experience, EST works better with a shorter tube. Do you have a measurement rig?
4. Crossover and acoustical design (including sound bore diameter and length, dampers, and even chambers that couple two or more drivers into one tube) are crucial. Overall, you can think about using BAs as a bandpass driver, DD with a lowpass, and EST works well only in high frequencies, so you'll need to use a highpass filter there. After that, it's all about trial and error and calculations to get the proper crossover points.
5. Everything depends on the signature you want to achieve. I would probably use a damper on the DD for low frequencies (like 2200 or 4700 ohms), and a smaller damper for the BA combo to smooth out the midrange. You can use a smaller ID tube for DD, and bigger for BAs and EST. EST does not require a damper, in my opinion, but with electrostatic drivers, acoustic design is everything - play with tube diameter, lengths, and well-calculated horns work well too. Don't forget to cross it relatively high too.
6. Search for PVC tubing at your local supplier or just order from AliExpress from Soundlink - they sell smaller quantities, such as 1m or so.
7. UV glue works best when it comes to placing drivers inside the shell. You can use Fotoplast Gel from Dreve, or a different UV glue with a lower viscosity, gel-like consistency (like Parlite 4760).
Fingers crossed for your build