[@Gisk]
1. In major cities magic would be common enough that its use is a staple of life. Yes everyone is capable of using magic, but if it's not trained or built up it remains relatively weak. So only those who actively work toward mastering magic can use it in a practical manner.

2. people are generally limited to their own element, although another element can be used with extreme training and focus. it is a rare thing to use another element using natural means, but through the use of Relics it is much easier and doable. 

3. An extracted soul stone will never grow stronger, but will kinda function as a battery having a max limit that as long as it isn't fully depleted will recharge to that limit. Example You have a wand that allows you to shoot a firebolt, that wand's soulstone allows you to fire a maximum of 5 firebolts a day. as long as you don't use all 5 you can keep using it in this way as it recharges it's energy between firing, but it will never get stronger increasing the limit to 6.