I am also experiencing not having the "2560x1440" for Half-Life. I do have this resolution for Blue Shift, so it is working there. I have tried previous methods mentioned above, no luck. I even unplugged all my displays except for my main and it still did not work. I do think that there might be something wrong with how the Half-Life patch is finding the resolutions, because I made a custom resolution at 4K and that shows up, but no 2560x1440. it seems like it is somehow skipping a resolution (or more). I know you said that Half-Life can only pre-cache the first 50 resolutions, but I have less than that. Even in your screenshot it does not show 2560x1440 where it should. Again, it works just fine for Blue Shift, just not in Half-Life even though they are both relatively the same.
I think you should still examine the patch.
Scottvordem
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