ymb,
1. It’s a sevara. The three shevuot are a package deal. If they do not persecute us “too much” we will not rebel. The Maharsha alludes to this. He also says that not making aliya “like a wall” refers to making aliya by force and building walled cities like Nehemia. The Maharal says (Chiddushei Aggadot) that the three together allow the Galut to exist. Thus, if one falls they all fall.
2. Your quote was obviously either written before the Balfour Declaration and San Remo conference or was simply a commentary on the pasuk. After these events he said that the walll had been broken down (“HaTekufa HaGedola” pg. 175 quoted in “Torat Eretz Yisrael” pg.289 footnote 28 and “HaTor” second publishing year, third edition quoted there pg. 234).