1) Form 3115 only pertains to the depreciation issue, and is the only permissible method for changing the amount of depreciation. You get a 481(a) adjustment in the current year (2020) for the 2018 and 2019 amounts, plus 2020 depreciation under the newly-adopted method.
2) The depreciation taken in 2018 and 2019 was correct under the taxpayer's adopted method of accounting. The accounting method change isn't retroactive; the prior-year adjustment is handled under 481(a).
3) No, the return has to be "true, correct, and complete" to "the best of [your] knowledge and belief." You can't start with an incorrect number (unless the client has already gotten a benefit from taking a larger deduction, in which case the prohibition on double-dipping wins). Note also that the statute of limitations for the IRS to examine the carryforward runs concurrent with the year in which the loss is eventually used, not the year in which it arose.
4) Yep. If it were an NOL, the statutory authority would be Rev Rul 81-88; I don't happen to have a Form 8275 handy for one in which a passive loss carryforward was involved.
5) The answer to "can I wink and nudge so my client can cheat" is "no." The answer to "can I make a good-faith reliance on my client's reasonable representation that they file the amended returns I prepared" is yes.
In the simple scenario in which both 2018 and 2019 were loss years, and that there was no current tax benefit from the losses (due to the AGI > $150k passive loss limitation, for instance), document the right amounts and carry the correct amount to 2020. I personally might file a Form 8275, particularly if the client is likely to change preparers before using up all the loss carry fowards, because there's nothing worse than a client coming along with all their back returns and there's no documentation as to why the carry forward coming out of the 2019 return isn't the same as the one coming into the 2020 return, and 10 years from now the client isn't going to have any idea.
In the complex scenario in which amending 2018 and/or 2019 resulted in a change in tax for one or both of those years, amend the years that change and carry the correct amount to 2020.
Reference document: http://www2.csudh.edu/rmalamud/tn083099.PDF is pretty readable, and has good footnote citations. There have been no substantive changes in the last 20 years; you may note that the references in the document go back to the 1960s, so this is pretty old stuff. It's just obscure.