Hey, I have that same plant in the same pot and media and all! I never took it out of its pot. just watered carefully when potting mix dries a little bit.
It grew two new pbs and both are pushing up spikes.
I do not repot unless they walk out of the pot.
I have no idea why your plant with seemingly good roots are on the decline.
You mentioned you water when the moss is still wet. I think one, you upset the plant by repotting it (when I repot oncidiums when they are not growing new shoots, they always sulk!) and two, you overwatered it apprently, you mentioned it yourself.
I actually think whatever media it originally comes in is pretty nice one. I try to imitate that by using peat or coir plus perlite. It is very moisture retentive and airy, which is perfect for oncidium.
As someone mentioned above, generally no orhids or plants unless if they are water plants, do not like to be wet all the time. Root rots occur, not just to orchids but all common houseplants, again, except for those that are supposed to be wet of course.
Oncidiums do best with constant moisture especially while growing otherwise, pb will wrinkle or growth won't reach to full&potential max, flowering won't be as great if any. This is different than being "wet" all the time.
Back to nursing your plant back to healthy state, well, I would move it to drier moss as the one it is in is too wet. pack lightly and wait.
Time will tell.
Cutting the spike at this point doesn't do anything. Leave it or cut it as you wish.
Oh, and the browing pb, I would cut it out before it spreads to other parts.
Did this start after you repot it??
Also, certain type of fungus can cause plant to dry up and die. just a possibility, not saying this is it.
wish the best for your plant!
Onc. Heaven scent - wrinkling, rotting spikes, the whole 9!
I am new to Oncidiums but have done research and know basic care. (questions highlighted for ease, Pictures related to all problems attached 
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