/-- Differentiation under integral of `x ↦ ∫ F x a` at a given point `x₀`, assuming `F x₀` is
integrable, `‖F x a - F x₀ a‖ ≤ bound a * ‖x - x₀‖` for `x` in a ball around `x₀` for ae `a` with
integrable Lipschitz bound `bound` (with a ball radius independent of `a`), and `F x` is
ae-measurable for `x` in the same ball. See `hasFDerivAt_integral_of_dominated_loc_of_lip` for a
slightly less general but usually more useful version. -/
theorem hasFDerivAt_integral_of_dominated_loc_of_lip' {F : H → α → E} {F' : α → H →L[𝕜] E} {x₀ : H}
    {bound : α → ℝ} {ε : ℝ} (ε_pos : 0 < ε)
    (hF_meas : ∀ x ∈ ball x₀ ε, AEStronglyMeasurable (F x) μ) (hF_int : Integrable (F x₀) μ)
    (hF'_meas : AEStronglyMeasurable F' μ)
    (h_lipsch : ∀ᵐ a ∂μ, ∀ x ∈ ball x₀ ε, ‖F x a - F x₀ a‖ ≤ bound a * ‖x - x₀‖)
    (bound_integrable : Integrable (bound : α → ℝ) μ)
    (h_diff : ∀ᵐ a ∂μ, HasFDerivAt (fun x => F x a) (F' a) x₀) :
    Integrable F' μ ∧ HasFDerivAt (fun x => ∫ a, F x a ∂μ) (∫ a, F' a ∂μ) x₀ := by
  have x₀_in : x₀ ∈ ball x₀ ε := mem_ball_self ε_pos
  have nneg : ∀ x, 0 ≤ ‖x - x₀‖⁻¹ := fun x => inv_nonneg.mpr (norm_nonneg _)
  set b : α → ℝ := fun a => |bound a|
  have b_int : Integrable b μ := bound_integrable.norm
  have b_nonneg : ∀ a, 0 ≤ b a := fun a => abs_nonneg _
  replace h_lipsch : ∀ᵐ a ∂μ, ∀ x ∈ ball x₀ ε, ‖F x a - F x₀ a‖ ≤ b a * ‖x - x₀‖
  exact h_lipsch.mono fun a ha x hx =>
    (ha x hx).trans <| mul_le_mul_of_nonneg_right (le_abs_self _) (norm_nonneg _)
  have hF_int' : ∀ x ∈ ball x₀ ε, Integrable (F x) μ := fun x x_in ↦ by
    have : ∀ᵐ a ∂μ, ‖F x₀ a - F x a‖ ≤ ε * b a := by
      simp only [norm_sub_rev (F x₀ _)]
      refine' h_lipsch.mono fun a ha => (ha x x_in).trans _
      rw [mul_comm ε]
      rw [mem_ball, dist_eq_norm] at x_in
      exact mul_le_mul_of_nonneg_left x_in.le (b_nonneg _)
    exact integrable_of_norm_sub_le (hF_meas x x_in) hF_int
      (bound_integrable.norm.const_mul ε) this
  have hF'_int : Integrable F' μ :=
    have : ∀ᵐ a ∂μ, ‖F' a‖ ≤ b a := by
      apply (h_diff.and h_lipsch).mono
      rintro a ⟨ha_diff, ha_lip⟩
      refine' ha_diff.le_of_lip' (b_nonneg a) (mem_of_superset (ball_mem_nhds _ ε_pos) <| ha_lip)
    b_int.mono' hF'_meas this
  refine' ⟨hF'_int, _⟩
  have h_ball : ball x₀ ε ∈ 𝓝 x₀ := ball_mem_nhds x₀ ε_pos
  have : ∀ᶠ x in 𝓝 x₀, ‖x - x₀‖⁻¹ * ‖((∫ a, F x a ∂μ) - ∫ a, F x₀ a ∂μ) - (∫ a, F' a ∂μ) (x - x₀)‖ =
      ‖∫ a, ‖x - x₀‖⁻¹ • (F x a - F x₀ a - F' a (x - x₀)) ∂μ‖ := by
    apply mem_of_superset (ball_mem_nhds _ ε_pos)
    intro x x_in; simp only
    rw [Set.mem_setOf_eq, ← norm_smul_of_nonneg (nneg _), integral_smul, integral_sub, integral_sub,
      ← ContinuousLinearMap.integral_apply hF'_int]
    exacts [hF_int' x x_in, hF_int, (hF_int' x x_in).sub hF_int,
      hF'_int.apply_continuousLinearMap _]
  rw [hasFDerivAt_iff_tendsto, tendsto_congr' this, ← tendsto_zero_iff_norm_tendsto_zero, ←
    show (∫ a : α, ‖x₀ - x₀‖⁻¹ • (F x₀ a - F x₀ a - (F' a) (x₀ - x₀)) ∂μ) = 0 by simp]
  apply tendsto_integral_filter_of_dominated_convergence
  · filter_upwards [h_ball] with _ x_in
    apply AEStronglyMeasurable.const_smul
    exact ((hF_meas _ x_in).sub (hF_meas _ x₀_in)).sub (hF'_meas.apply_continuousLinearMap _)
  · refine mem_of_superset h_ball fun x hx ↦ ?_
    apply (h_diff.and h_lipsch).mono
    rintro a ⟨-, ha_bound⟩
    show ‖‖x - x₀‖⁻¹ • (F x a - F x₀ a - F' a (x - x₀))‖ ≤ b a + ‖F' a‖
    replace ha_bound : ‖F x a - F x₀ a‖ ≤ b a * ‖x - x₀‖ := ha_bound x hx
    calc
      ‖‖x - x₀‖⁻¹ • (F x a - F x₀ a - F' a (x - x₀))‖ =
          ‖‖x - x₀‖⁻¹ • (F x a - F x₀ a) - ‖x - x₀‖⁻¹ • F' a (x - x₀)‖ :=
        by rw [smul_sub]
      _ ≤ ‖‖x - x₀‖⁻¹ • (F x a - F x₀ a)‖ + ‖‖x - x₀‖⁻¹ • F' a (x - x₀)‖ := (norm_sub_le _ _)
      _ = ‖x - x₀‖⁻¹ * ‖F x a - F x₀ a‖ + ‖x - x₀‖⁻¹ * ‖F' a (x - x₀)‖ := by
        rw [norm_smul_of_nonneg, norm_smul_of_nonneg] <;> exact nneg _
      _ ≤ ‖x - x₀‖⁻¹ * (b a * ‖x - x₀‖) + ‖x - x₀‖⁻¹ * (‖F' a‖ * ‖x - x₀‖) := by
        gcongr; exact (F' a).le_op_norm _
      _ ≤ b a + ‖F' a‖ := ?_
    simp only [← div_eq_inv_mul]
    apply_rules [add_le_add, div_le_of_nonneg_of_le_mul] <;> first | rfl | positivity
  · exact b_int.add hF'_int.norm
  · apply h_diff.mono
    intro a ha
    suffices Tendsto (fun x => ‖x - x₀‖⁻¹ • (F x a - F x₀ a - F' a (x - x₀))) (𝓝 x₀) (𝓝 0) by simpa
    rw [tendsto_zero_iff_norm_tendsto_zero]
    

Proof state:
case h_lim
α : Type u_1
inst✝⁷ : MeasurableSpace α
μ : Measure α
𝕜 : Type u_2
inst✝⁶ : IsROrC 𝕜
E : Type u_3
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℝ E
inst✝³ : NormedSpace 𝕜 E
inst✝² : CompleteSpace E
H : Type u_4
inst✝¹ : NormedAddCommGroup H
inst✝ : NormedSpace 𝕜 H
F : H → α → E
F' : α → H →L[𝕜] E
x₀ : H
bound : α → ℝ
ε : ℝ
ε_pos : 0 < ε
hF_meas : ∀ x ∈ ball x₀ ε, AEStronglyMeasurable (F x) μ
hF_int : Integrable (F x₀)
hF'_meas : AEStronglyMeasurable F' μ
bound_integrable : Integrable bound
h_diff : ∀ᵐ (a : α) ∂μ, HasFDerivAt (fun x => F x a) (F' a) x₀
x₀_in : x₀ ∈ ball x₀ ε
nneg : ∀ (x : H), 0 ≤ ‖x - x₀‖⁻¹
b : α → ℝ := fun a => |bound a|
b_int : Integrable b
b_nonneg : ∀ (a : α), 0 ≤ b a
h_lipsch : ∀ᵐ (a : α) ∂μ, ∀ x ∈ ball x₀ ε, ‖F x a - F x₀ a‖ ≤ b a * ‖x - x₀‖
hF_int' : ∀ x ∈ ball x₀ ε, Integrable (F x)
hF'_int : Integrable F'
h_ball : ball x₀ ε ∈ 𝓝 x₀
this :
  ∀ᶠ (x : H) in 𝓝 x₀,
    ‖x - x₀‖⁻¹ * ‖∫ (a : α), F x a ∂μ - ∫ (a : α), F x₀ a ∂μ - (∫ (a : α), F' a ∂μ) (x - x₀)‖ =
      ‖∫ (a : α), ‖x - x₀‖⁻¹ • (F x a - F x₀ a - (F' a) (x - x₀)) ∂μ‖
a : α
ha : HasFDerivAt (fun x => F x a) (F' a) x₀
⊢ Tendsto (fun x => ‖‖x - x₀‖⁻¹ • (F x a - F x₀ a - (F' a) (x - x₀))‖) (𝓝 x₀) (𝓝 0)

Next tactic:
have : (fun x => ‖x - x₀‖⁻¹ * ‖F x a - F x₀ a - F' a (x - x₀)‖) = fun x =>
        ‖‖x - x₀‖⁻¹ • (F x a - F x₀ a - F' a (x - x₀))‖ := by
      ext x
      rw [norm_smul_of_nonneg (nneg _)]