How a particular use of Evocation physically manifests is up to the individual user and how their own philosophy of magic operates. Harry's basic Fuego spell, for example, is typically a glowing red beam of heat/fire. Other wizards' attacks look like starbursts or fireworks. A fire shield might look like a barrier of flame, or a glowing red bubble around you.
The element in question most certainly does not have to be present. Ramierez' specialty is water, for example, and his standard attack spell appears from nowhere as a silent green blast that disintegrates his target. Thinking about it in terms of "bending" is appropriate for a particular practitioner's way of viewing his magic, but it's not at all definitive for how all wizards would use their power. You can absolutely have the effects you describe for a shield.
One thing which is fairly constant, or seems to be, is that anything manifested by magic (blasts or fire, water, etc) aren't really those things. They're formed from energy and dissipate shortly after the spell is cast. You wouldn't like find lots of water lying around after ware evocations were used, but objects might show signs of water damage, like warped paper or wood.
Hexing doesn't seem to be required to be tied to a particular element. It just makes things go wrong with technology. You could add some flavour to it that way if you wanted, though, I imagine.
