The warranty is not on each part, it is on the unit as a whole. If a part goes bad and renders the amp defective, we replace the part under warranty. If there are 400 parts in the amp, there are not 400 warranties. You can't void part of the warranty and keep the rest.
As a warranty repair tech, it is up to me to decide when I see it. SO if I see some mod like that in an amp, and it appears well done, and does not seem to be involved with the problem, I might ignore it. I might not.
Just my opinion, but the main usefulness of a half-power switch is in marketing the amp. Otherwise, I don't see a lot of use for it. The volume reduction is only 3 decibels, maybe less if the B+ rises under the reduced load. Might be a little easier to break up the remaining power tubes if you are going for that, but mainly you lose headroom.
As to why they didn't add one, I suspect the added cost would not be made up in increased sales. They'd have to design a new power tube board and make it, plus make room on the panel for the switch and wire it. Or maybe do it with relays.
WHen you pull tubes, you also SHOULD change the impedance tap used for the speakers. I don't think it matters much, as no one I know who has a half-power switch bothers doing so.