Useless? No, Parrying still gives defense without a shield, only less. But still, it won't do much in the late game. Maybe like 2-3% less chance of being hit at best. Especially on a Gladiator, because you will have Heroic Courage running all the time, providing you with several thousand % of attack/defense bonuses. So a couple hundred % more from Parrying doesn't really mean much. I just picked it because I had nothing else to pick. Sitting at 1 skill point and letting items do the rest.
As for Tree Rage, it is not exclusive to Disarming and can also drop from other bosses. I got 2 of them shortly after each other (both level 165) and never touched Disarming. One was from a Dragon, the other one I don't remember as it's been months since I last played. So unless it's the 2-handed version that is exclusive to Disarming, I can safely say that it is entirely possible to get the 1 handed version without Disarming. Here is even a screen as proof of the two I got and the skills I have on my Gladiator:
Also keep in mind, that the 1 handed Tree Rage requires Axe Lore.
I don't think any item has only one source. I got Thain's Axe from a drop. I got Hockey Mask from a drop. Items where all sources claim can be got only once from a pre-determined location. I was playing the game in the purely vanilla version, with no mods/community patches installed.