How can I keep playing online-only Flash games after the Flash shutdown in 2020?

Unfortunately, there's no current (legal) way to disable the Adobe Flash killswitch, and I would obviously not recommend trying to illegally "crack" copyrighted games unless you want angry lawyers coming your way. (It would also require a great deal of programming experience.) The Flash shutdown really, really sucks for older Flash games, including some of my own favorites.

However, many clever and wonderful people have been working on archiving old Flash games, and some game companies have released non-Flash updates in order to preserve their games.

For Farmville 2...

It sounds like you've already discovered Farmville 2 Launcher +, which is Zynga's official solution to the impending Farmville 2 shutdown. This is the only official way to continue playing Farmville 2 after the Flash shutdown occurs.

The launcher states that these are the supported platforms - possibly your issue is that you're trying to run it on Internet Explorer or a non-supported version of Windows?

The FV2 Launcher+ is currently available only to users of Windows 7 and 10, on Chrome and Firefox browsers only. We are working on making it available to Mac users and on other browsers. It is available on Facebook & Zyngagames.com too!.

If you're having trouble installing the launcher, they also provide an installer walkthrough about how to download and open the file and install the game, including a video tutorial.

Installation tutorial:

For other games...

There are tons of great projects that are archiving old Flash games so that they can work without Flash, in case you are interested in checking out other old Flash games. Some of the ones I've seen are Flash Game Archive and Internet Archive. The Flash Game Archive in particular now supports local offline play for many of its games.

Disclaimer: This suggestion is dangerous and can leave you vulnerable to security exploits.

You can download an old version of Firefox which does not have the Flash killswitch from https://ftp.mozilla.org/pub/firefox/releases/

You can download an old Flash version which does not have the killswitch from https://adobe_flash_player.en.downloadastro.com/old_versions/

Avoid installing the KB4577586 update on your machine. According to https://www.bleepingcomputer.com/news/microsoft/microsoft-releases-update-to-remove-adobe-flash-from-windows/ the update is optional and only available via the Microsoft Catalog so I'm not sure if that means Windows will not automatically apply it.

Preferably, you would do this all in a virtual machine dedicated to running Farmville 2.

In the end this will probably be fruitless as Zynga themselves will likely remove the Flash version of Farmville 2 from the Internet.

Although you may not be able to, it's possible the websites themselves will be able to, using the Rust language project called Ruffle, a Flash Emulator. You described it as a local-emulator, but that's not all it is.

Ruffle is a Flash Player emulator written in Rust. Ruffle runs natively on all modern operating systems as a standalone application, and on all modern browsers through the use of WebAssembly. Leveraging the safety of the modern browser sandbox and the memory safety guarantees of Rust, we can confidently avoid all the security pitfalls that Flash had a reputation for. Ruffle puts Flash back on the web, where it belongs - including iOS and Android!

Designed to be easy to use and install, users or website owners may install the web version of Ruffle and existing flash content will "just work", with no extra configuration required. Ruffle will detect all existing Flash content on a website and automatically "polyfill" it into a Ruffle player, allowing seamless and transparent upgrading of websites that still rely on Flash content.

Originally I only thought this was worth a comment. But looking at it again - if major sites like Farmville are Flash-only, it seems like they'd have a major incentive to keep their sites running. So that's one way you could keep playing online-only games, if they switch over to Ruffle.