When I try to install Flash Player in Mojave using either Safari or Firefox
I go through the first two steps of the download and get to the third step,
which brings up a try Adobe Stock screen and there is no way to get past that
screen that I can see. How do I decline the trial and get the install to proceed,
or what other workaround is there
When you arrive at step 3, it should say (in small print) that the install of Flash Player has finished, and worked fine. This is now an advert you can ignore, no need to respond.
So, good news, Flash is installed for you. Of course it may not work but that's something to ask us about.
The installer file downloads at step 2. The default location is the Downloads directory. After you install Flash Player, the default browser automatically launches to step 3, which confirms the installation and displays various offers, in this case, it's an Adobe Stock offer. These offers are completely optional and have no bearing on Flash Player installation.