No, you never know when you will loose the confinemen; so you don’t want to make the member critical under such condiment loss. Only in very specific applications when it is guaranteed that the confinment won’t be lost during the intended service life of the member.

what do u mean by a confined column or wall? Full confinement needs continuous spiral in the column, and the code acknowledges this by increasing the strength reduction factor for spirally confined columns.

For the wall, how you classify it as confined? You need a lot of reinforcement two layers in each direction with close spacing between the bars.

so to achieve full confinement, it will cost more than the use of unconfined strength as per current codes

this value is so good but in lab situations and in applying at real structure its very hard to reach to this result. however if you reached to that aim ,u have to consider the unexpected yield against absorbing quaking forces and immediately breaking cz of concrete

I suggest you obtain ACI's SP-238: International Symposium on Confined Concrete. Published in 2006, it contains 28 papers on the subject.

It all depends on the percentage trust on the value obtained. There is nothing wrong in using the value. It is often best to take care of unforseen cases but if not one has to bear in mind the difference between laboratory controlled experiments and the behaviour of structures under real conditions.