1) Yes, once you've adopted a method of accounting, even if it's an incorrect / impermissible method, a 3115 is the only technically correct way to change your method of accounting. Not every decision constitutes a method of accounting, but using the same depreciation method ("no depreciation" is a method) for the first two years an asset is placed in service constitutes adopting a method of accounting with respect to that asset.
2) The client can choose to amend 2019, or not. The amount carried forward into 2020 is required to be the "as if prior returns had been prepared correctly" amount, whether the client chooses to amend 2019 or not. "Correctly" in this case includes "using the taxpayer's adopted method of accounting," so isn't a workaround for the depreciation issue.
Caveat that if the client got an impermissible benefit in a prior year, you can't double-dip. So if your client had taken an impermissible rental loss in 2019, the amount carried forward to 2020 would be reduced by that impermissible loss unless the client amended 2019.