theorem exists_normalized_aux3 {N : ℕ} {τ : ℝ} (a : SatelliteConfig E N τ)
    (lastc : a.c (last N) = 0) (lastr : a.r (last N) = 1) (hτ : 1 ≤ τ) (δ : ℝ) (hδ1 : τ ≤ 1 + δ / 4)
    (i j : Fin N.succ) (inej : i ≠ j) (hi : 2 < ‖a.c i‖) (hij : ‖a.c i‖ ≤ ‖a.c j‖) :
    1 - δ ≤ ‖(2 / ‖a.c i‖) • a.c i - (2 / ‖a.c j‖) • a.c j‖ := by
  have ah :
      ∀ i j, i ≠ j → a.r i ≤ ‖a.c i - a.c j‖ ∧ a.r j ≤ τ * a.r i ∨
        a.r j ≤ ‖a.c j - a.c i‖ ∧ a.r i ≤ τ * a.r j := by
    simpa only [dist_eq_norm] using a.h
  have δnonneg : 0 ≤ δ := by linarith only [hτ, hδ1]
  have hcrj : ‖a.c j‖ ≤ a.r j + 1 := by simpa only [lastc, lastr, dist_zero_right] using a.inter' j
  have A : a.r i ≤ ‖a.c i‖ := by
    have : i < last N := by
      apply lt_top_iff_ne_top.2
      intro iN
      change i = last N at iN
      rw [iN, lastc, norm_zero] at hi
      exact lt_irrefl _ (zero_le_two.trans_lt hi)
    convert (a.hlast i this).1 using 1
    rw [dist_eq_norm, lastc, sub_zero]
  have hj : 2 < ‖a.c j‖ := hi.trans_le hij
  set s := ‖a.c i‖
  have spos : 0 < s := zero_lt_two.trans hi
  set d := (s / ‖a.c j‖) • a.c j with hd
  have I : ‖a.c j - a.c i‖ ≤ ‖a.c j‖ - s + ‖d - a.c i‖ :=
    calc
      ‖a.c j - a.c i‖ ≤ ‖a.c j - d‖ + ‖d - a.c i‖ := by simp [← dist_eq_norm, dist_triangle]
      _ = ‖a.c j‖ - ‖a.c i‖ + ‖d - a.c i‖ := by
        nth_rw 1 [← one_smul ℝ (a.c j)]
        rw [add_left_inj, hd, ← sub_smul, norm_smul, Real.norm_eq_abs, abs_of_nonneg, sub_mul,
          one_mul, div_mul_cancel _ (zero_le_two.trans_lt hj).ne']
        rwa [sub_nonneg, div_le_iff (zero_lt_two.trans hj), one_mul]
  have J : a.r j - ‖a.c j - a.c i‖ ≤ s / 2 * δ :=
    calc
      a.r j - ‖a.c j - a.c i‖ ≤ s * (τ - 1) := by
        rcases ah j i inej.symm with (H | H)
        · calc
            a.r j - ‖a.c j - a.c i‖ ≤ 0 := sub_nonpos.2 H.1
            _ ≤ s * (τ - 1) := mul_nonneg spos.le (sub_nonneg.2 hτ)
        · rw [norm_sub_rev] at H
          calc
            a.r j - ‖a.c j - a.c i‖ ≤ τ * a.r i - a.r i := sub_le_sub H.2 H.1
            _ = a.r i * (τ - 1) := by ring
            _ ≤ s * (τ - 1) := mul_le_mul_of_nonneg_right A (sub_nonneg.2 hτ)
      _ ≤ s * (δ / 2) := (mul_le_mul_of_nonneg_left (by linarith only [δnonneg, hδ1]) spos.le)
      _ = s / 2 * δ := by ring
  have invs_nonneg : 0 ≤ 2 / s := div_nonneg zero_le_two (zero_le_two.trans hi.le)
  calc
    1 - δ = 2 / s * (s / 2 - s / 2 * δ) := by field_simp [spos.ne']; ring
    _ ≤ 2 / s * ‖d - a.c i‖ :=
      (mul_le_mul_of_nonneg_left (by linarith only [hcrj, I, J, hi]) invs_nonneg)
    _ = ‖(2 / s) • a.c i - (2 / ‖a.c j‖) • a.c j‖ := by
      conv_lhs => rw [norm_sub_rev, ← abs_of_nonneg invs_nonneg]
      

Proof state:
E : Type u_1
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
N : ℕ
τ : ℝ
a : SatelliteConfig E N τ
lastc : c a (last N) = 0
lastr : r a (last N) = 1
hτ : 1 ≤ τ
δ : ℝ
hδ1 : τ ≤ 1 + δ / 4
i j : Fin (Nat.succ N)
inej : i ≠ j
ah :
  ∀ (i j : Fin (Nat.succ N)),
    i ≠ j → r a i ≤ ‖c a i - c a j‖ ∧ r a j ≤ τ * r a i ∨ r a j ≤ ‖c a j - c a i‖ ∧ r a i ≤ τ * r a j
δnonneg : 0 ≤ δ
hcrj : ‖c a j‖ ≤ r a j + 1
hj : 2 < ‖c a j‖
s : ℝ := ‖c a i‖
hi : 2 < s
hij : s ≤ ‖c a j‖
A : r a i ≤ s
spos : 0 < s
d : E := (s / ‖c a j‖) • c a j
hd : d = (s / ‖c a j‖) • c a j
I : ‖c a j - c a i‖ ≤ ‖c a j‖ - s + ‖d - c a i‖
J : r a j - ‖c a j - c a i‖ ≤ s / 2 * δ
invs_nonneg : 0 ≤ 2 / s
⊢ |2 / s| * ‖c a i - d‖ = ‖(2 / s) • c a i - (2 / ‖c a j‖) • c a j‖

Next tactic:
rw [← Real.norm_eq_abs, ← norm_smul, smul_sub, hd, smul_smul]