Gid, forgive me if I am misunderstanding what is wrong with your M8 but I'm not sure how your RF is misaligned in a way that is only apparent when you do the focus test. I presume you are not confusing the effects of parallax when trying to align two edges at an angle in the close range (in the rangefinder patch the edges appear at different angles and will only align at the point where they cross). This is normal.
However, backfocussing is another issue and seems to be quite common. My own M8 is currently in Solms and is being corrected for this. The 2mm allen key fix looks seductively easy but, from my understanding, is possibly not the right adjustment for close range back focus (which my M8 suffers from). Messing with this adjustment isn't going to void your warranty (how would Leica know what you have done?) but it shouldn't really be necessary to tinker in this way with a £3k camera body to get proper focus with £1.5k lenses. If you have lost confidence with the M8 I think that you are right to consider getting a refund now rather than try and secure one later. You can always get another M8 later in the year.