theorem exists_forall_closed_ball_dist_add_le_two_sub (hε : 0 < ε) :
    ∃ δ, 0 < δ ∧ ∀ ⦃x : E⦄, ‖x‖ ≤ 1 → ∀ ⦃y⦄, ‖y‖ ≤ 1 → ε ≤ ‖x - y‖ → ‖x + y‖ ≤ 2 - δ := by
  have hε' : 0 < ε / 3 := div_pos hε zero_lt_three
  obtain ⟨δ, hδ, h⟩ := exists_forall_sphere_dist_add_le_two_sub E hε'
  set δ' := min (1 / 2) (min (ε / 3) <| δ / 3)
  refine' ⟨δ', lt_min one_half_pos <| lt_min hε' (div_pos hδ zero_lt_three), fun x hx y hy hxy => _⟩
  obtain hx' | hx' := le_or_lt ‖x‖ (1 - δ')
  · rw [← one_add_one_eq_two]
    exact (norm_add_le_of_le hx' hy).trans (sub_add_eq_add_sub _ _ _).le
  obtain hy' | hy' := le_or_lt ‖y‖ (1 - δ')
  · rw [← one_add_one_eq_two]
    exact (norm_add_le_of_le hx hy').trans (add_sub_assoc _ _ _).ge
  have hδ' : 0 < 1 - δ' := sub_pos_of_lt (min_lt_of_left_lt one_half_lt_one)
  have h₁ : ∀ z : E, 1 - δ' < ‖z‖ → ‖‖z‖⁻¹ • z‖ = 1 := by
    rintro z hz
    rw [norm_smul_of_nonneg (inv_nonneg.2 <| norm_nonneg _), inv_mul_cancel (hδ'.trans hz).ne']
  have h₂ : ∀ z : E, ‖z‖ ≤ 1 → 1 - δ' ≤ ‖z‖ → ‖‖z‖⁻¹ • z - z‖ ≤ δ' := by
    rintro z hz hδz
    nth_rw 3 [← one_smul ℝ z]
    rwa [← sub_smul, norm_smul_of_nonneg (sub_nonneg_of_le <| one_le_inv (hδ'.trans_le hδz) hz),
      sub_mul, inv_mul_cancel (hδ'.trans_le hδz).ne', one_mul, sub_le_comm]
  set x' := ‖x‖⁻¹ • x
  set y' := ‖y‖⁻¹ • y
  have hxy' : ε / 3 ≤ ‖x' - y'‖ :=
    calc
      ε / 3 = ε - (ε / 3 + ε / 3) := by ring
      _ ≤ ‖x - y‖ - (‖x' - x‖ + ‖y' - y‖) := by
        gcongr
        · exact (h₂ _ hx hx'.le).trans <| min_le_of_right_le <| min_le_left _ _
        · exact (h₂ _ hy hy'.le).trans <| min_le_of_right_le <| min_le_left _ _
      _ ≤ _ := by
        have : ∀ x' y', x - y = x' - y' + (x - x') + (y' - y) := fun _ _ => by abel
        rw [sub_le_iff_le_add, norm_sub_rev _ x, ← add_assoc, this]
        exact norm_add₃_le _ _ _
  calc
    ‖x + y‖ ≤ ‖x' + y'‖ + ‖x' - x‖ + ‖y' - y‖ := by
      have : ∀ x' y', x + y = x' + y' + (x - x') + (y - y') := fun _ _ => by abel
      rw [norm_sub_rev, norm_sub_rev y', this]
      

Proof state:
E : Type u_1
inst✝² : SeminormedAddCommGroup E
inst✝¹ : UniformConvexSpace E
ε : ℝ
inst✝ : NormedSpace ℝ E
hε : 0 < ε
hε' : 0 < ε / 3
δ : ℝ
hδ : 0 < δ
h : ∀ ⦃x : E⦄, ‖x‖ = 1 → ∀ ⦃y : E⦄, ‖y‖ = 1 → ε / 3 ≤ ‖x - y‖ → ‖x + y‖ ≤ 2 - δ
δ' : ℝ := min (1 / 2) (min (ε / 3) (δ / 3))
x : E
hx : ‖x‖ ≤ 1
y : E
hy : ‖y‖ ≤ 1
hxy : ε ≤ ‖x - y‖
hx' : 1 - δ' < ‖x‖
hy' : 1 - δ' < ‖y‖
hδ' : 0 < 1 - δ'
h₁ : ∀ (z : E), 1 - δ' < ‖z‖ → ‖‖z‖⁻¹ • z‖ = 1
h₂ : ∀ (z : E), ‖z‖ ≤ 1 → 1 - δ' ≤ ‖z‖ → ‖‖z‖⁻¹ • z - z‖ ≤ δ'
x' : E := ‖x‖⁻¹ • x
y' : E := ‖y‖⁻¹ • y
hxy' : ε / 3 ≤ ‖x' - y'‖
this : ∀ (x' y' : E), x + y = x' + y' + (x - x') + (y - y')
⊢ ‖?x' + ?y' + (x - ?x') + (y - ?y')‖ ≤ ‖x' + y'‖ + ‖x - x'‖ + ‖y - y'‖
case x'
E : Type u_1
inst✝² : SeminormedAddCommGroup E
inst✝¹ : UniformConvexSpace E
ε : ℝ
inst✝ : NormedSpace ℝ E
hε : 0 < ε
hε' : 0 < ε / 3
δ : ℝ
hδ : 0 < δ
h : ∀ ⦃x : E⦄, ‖x‖ = 1 → ∀ ⦃y : E⦄, ‖y‖ = 1 → ε / 3 ≤ ‖x - y‖ → ‖x + y‖ ≤ 2 - δ
δ' : ℝ := min (1 / 2) (min (ε / 3) (δ / 3))
x : E
hx : ‖x‖ ≤ 1
y : E
hy : ‖y‖ ≤ 1
hxy : ε ≤ ‖x - y‖
hx' : 1 - δ' < ‖x‖
hy' : 1 - δ' < ‖y‖
hδ' : 0 < 1 - δ'
h₁ : ∀ (z : E), 1 - δ' < ‖z‖ → ‖‖z‖⁻¹ • z‖ = 1
h₂ : ∀ (z : E), ‖z‖ ≤ 1 → 1 - δ' ≤ ‖z‖ → ‖‖z‖⁻¹ • z - z‖ ≤ δ'
x' : E := ‖x‖⁻¹ • x
y' : E := ‖y‖⁻¹ • y
hxy' : ε / 3 ≤ ‖x' - y'‖
this : ∀ (x' y' : E), x + y = x' + y' + (x - x') + (y - y')
⊢ E
case y'
E : Type u_1
inst✝² : SeminormedAddCommGroup E
inst✝¹ : UniformConvexSpace E
ε : ℝ
inst✝ : NormedSpace ℝ E
hε : 0 < ε
hε' : 0 < ε / 3
δ : ℝ
hδ : 0 < δ
h : ∀ ⦃x : E⦄, ‖x‖ = 1 → ∀ ⦃y : E⦄, ‖y‖ = 1 → ε / 3 ≤ ‖x - y‖ → ‖x + y‖ ≤ 2 - δ
δ' : ℝ := min (1 / 2) (min (ε / 3) (δ / 3))
x : E
hx : ‖x‖ ≤ 1
y : E
hy : ‖y‖ ≤ 1
hxy : ε ≤ ‖x - y‖
hx' : 1 - δ' < ‖x‖
hy' : 1 - δ' < ‖y‖
hδ' : 0 < 1 - δ'
h₁ : ∀ (z : E), 1 - δ' < ‖z‖ → ‖‖z‖⁻¹ • z‖ = 1
h₂ : ∀ (z : E), ‖z‖ ≤ 1 → 1 - δ' ≤ ‖z‖ → ‖‖z‖⁻¹ • z - z‖ ≤ δ'
x' : E := ‖x‖⁻¹ • x
y' : E := ‖y‖⁻¹ • y
hxy' : ε / 3 ≤ ‖x' - y'‖
this : ∀ (x' y' : E), x + y = x' + y' + (x - x') + (y - y')
⊢ E

Next tactic:
exact norm_add₃_le _ _ _