Only after purchasing the certificate did I learn that installing it and making one’s site compatible with it is a somewhat involved process.

Not really, just make this one change to wp-config.php: https://codex.wordpress.org/Administration_Over_SSL#To_Force_SSL_Logins_and_SSL_Admin_Access

Moreover I am unimpressed by the reviews of the plugins that supposedly do much of this for me.

No need for a plugin, just the single line above. The plugin is for use if that single line doesn’t work out for you.

The supposed boost in Google page rank due to SSL appears to be more based on assertion than actual data.

Considering that Google is the one claiming you’ll get a boost from it, it’s probably best to take that as fact rather than simple assertion.

1. What is the worst that could happen to my site without SSL?

Less likely: One of your member’s accounts (and payment details, if you do that) will be compromised when they log in over unsecured wifi (a coffee shop, library, hotel, etc).

More likely: You’ll lose the trust of your members by forcing them to transmit login details (and payment details, if you do that) over an unsecured connection.

Would you use SSL if you were running a similar web site for your business?

Without a second thought, yes.

I’ve spent considerable time reading other’s experiences converting WP to SSL and it appears that many administrative issues can arise, including those which require a good knowledge of the guts of WordPress to resolve.

If you do it right, you should have no problem. I’ve had all of my sites setup with the single line at https://codex.wordpress.org/Administration_Over_SSL#To_Force_SSL_Logins_and_SSL_Admin_Access for years and have had no problems.

You’re welcome!