Ugh! smh.
I fully understand that attitude of "if it ain't broke..." 👍🏽
I tend to be just a little bit of a risk taker, myself. Things generally work out for the best, fortunately.
Re: the damaged installer... try a restart in Safe mode, then launch the installer while still in safe mode. If you get the same warning, delete the Mojave installer and retry the download again (still in safe mode).
And, by the way, if you haven't rebooted into safe mode anytime recently you should try that to see if it helps with the trackpad issue. A safe mode restart will let the Mac OS do drive directory repair, cache clearing and other misc housekeeping. Often just booting into safe mode and logging in, then immediately rebooting normally will resolve an issue. You should do this with all your external peripherals disconnected but the keyboard and mouse. Press the Shift key until you see the Apple logo and progress bar when you restart you Mac.
Booting into Safe Mode…Â
Use safe mode to isolate issues - Apple Support
Note– starting up in Safe mode could take 10 minutes or more. Be patient.
Does the problem persist while in Safe mode?Â
Restart your Macintosh and re-evaluate the issue.