That's a lot of questions luminous.
I made 4 devices but I am not sure which should I use or are they made correctly. I know it is possible for one device to connect with the other. Is anyone willing to ask that their device for me?
I would start out with one device. That way there is no confusion. Pick the one you feel the most drawn to.
Also, are symbols on the device necessary? (I didn't wrote them) And I am not sure where should I put my device.
I have read the symbols are not necessary. I put them on because I remembered about them during construction.
Can you connect with the device if you put it behind a pillow you're head is on, or your hands must be on the sides of the device? It is difficult for my hands to be constantly in that position on the side, can I just lay them down? Is it better to lay down with the device or just sit and put it on a table or something?
I think the position you are in is by personal choice. I use a lot of moisturizers on my hands especially during the winter and was afraid I would "dirty" my meditation device with a build-up of the ointments I use. My device assured me that was not the case. I tried holding mine and seemed to have a better connection rather than holding my hands on either side. My device does not seem to be interrupted by touching it. And sometimes it instructs me to put it at key points on my body for healing, cleansing or a buildup of energy.
And for those who have activated their devices, how quickly can you connect?
That all depends on you. You must believe in your device. Understand that it is a conduit for your higher self to communicate with you. And just as importantly, you must "trust" your device. Say it out loud.
Next, can you put a pillow or a bedsheet on the device and your hands on that, or will those materials block the energy coming from the device to your hands?
As with the dilemma of the ointments, I would think, but can't be sure, the bed sheet would be just fine.
Is the exact number of outer shells in the design really necessary (4 outer shells and 2 on sides) or can you put them as many as you can? Is it important that the cone shells be small for directing energy or not (I think I felt more of the energy when I didn't use such small cone shells like turritella but bigger ones)? Do sizes and proportions of shells matter?
I made some alterations during my construction as well. And I "felt" it was right for me and my device. I believe because I made my personal construction my way rather than the original instruction, I had a stronger connection to my device and made my contact more successful.
I think its important for the core shell to represent the golden ratio but what about other shells?
I used old shells that I had cleansed. Nothing special about them.
How much have you used the device before you managed to activate it?
Mine activated the first time I used it.
Did you meditate daily before using the device at all?
No
Will the device answer any question you ask?
My meditations with my device are not about questions and answers. It's about connection and I use mine a lot to heal and re-energize my body.
How do you connect with the device and how does it feel, and do you need to go to sleep and wake up in a lucid one to connect or can you just close your eyes and connect after a few seconds/minutes, while still having the perception of physical reality when you open your eyes?
I use my device when I go to bed, just before I go to sleep. My device does not connect strongly with me every time. And it connects differently with me each time. One time I needed healing with a kidney and it sent a black smoke into my body to the afflicted kidney, last night was a light energizing. It felt like a golden dust entering my body. It is different each time.
Did anybody put earth in the core shell instead of sand?
I did not. I used sea sand.
Does anybody know how to make a pendant device?
No. Here is a pendant made in Australia: 
Source: http://baramayaustralia.com.au/
Also, how to make this:
https://www.youtube.com/watch?v=HGnH_SpMsm0 ?
I have watched several videos by this person. I do not know how to make his device.
Did anybody try using more devices at the same time?
No, it was never necessary. One device was enough for me.
Does it matter if you put somebody else's hair in the device and use it? Will you then still talk with your higher self or does else's hair make some difference?
Someone else said it does not matter whose hair is in the device. It just has to be human hair from the head. I personally wanted my own hair in my device. I'm glad I did. It was one of my personal choices in the construction of my device.
Are there any special commands you can use so the device increases your clairvoyance and seeing with your third eye? How long does it usually take?
There are special commands but you can develop your own as you get to know and feel comfortable with your device. You'd have to research what I've abbreviated at the beginning of this thread and read what the experts outline as far as commands.
Each use with my device is different. It isn't a spiral or ladder which you climb higher and higher. Every day is different. Some days there is a lot of love emitting from my device, some days just a trickle.
The only additional advice I can give you is to dive in and experience it. As you trust and connect with your device, it will instruct you on the operation of it.