High Sierra to Mojave on mid-2011 Lion?

You can use the computer until it fails, Apple continues to update the security on it so you should be okay. However they normally cease that after awhile, my guess is you have about another 1-2 years before they quit updating.

Before you need a new Mac ensure you backup your computer every day with Time Machine and/or by creating a bootable clone. This safeguards your data in the event of a hardware failure which could happen ANY time.

Also, make it a habit to keep your apps (especially 3rd party apps) up-to-date. When it is time for the new Mac you can migration your data, settings and apps. However, some apps (any 32bit versions) will not migrate and some the developers will not allow to be migrated, Adobe and Microsoft are famous for that. This means they must be manually installed so safeguard any security codes for these apps and DVDs you have as you may need them.

It's not possible. A pre-2012 Mac(other than the 2010 desktop Mac Pro) can't run Mojave.

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Mojave requires at a minimum a GPU with Metal, this did not become available on iMacs until 2012. Sorry, no there is no work around. Your 2011 at HS is as far as it can go.