We are left with following problem, upon which TcT provides the
certificate YES(?,O(n^1)).

Strict Trs:
  { rev(rev(x)) -> x
  , rev(nil()) -> nil()
  , rev(++(x, y)) -> ++(rev(y), rev(x))
  , ++(x, nil()) -> x
  , ++(x, ++(y, z)) -> ++(++(x, y), z)
  , ++(nil(), y) -> y
  , ++(.(x, y), z) -> .(x, ++(y, z))
  , make(x) -> .(x, nil()) }
Obligation:
  innermost runtime complexity
Answer:
  YES(?,O(n^1))

The problem is match-bounded by 1. The enriched problem is
compatible with the following automaton.
{ rev_0(2) -> 1
, nil_0() -> 1
, nil_0() -> 2
, nil_0() -> 3
, nil_1() -> 1
, ++_0(2, 2) -> 1
, ++_1(2, 2) -> 3
, ._0(2, 2) -> 1
, ._0(2, 2) -> 2
, ._0(2, 2) -> 3
, ._1(2, 1) -> 1
, ._1(2, 3) -> 1
, ._1(2, 3) -> 3
, make_0(2) -> 1 }

Hurray, we answered YES(?,O(n^1))