Sometimes in the haste of trying to diagnose problems I/we put on blinders and miss the obvious.
The "lights on/clicking starter" is often a symptom of simply a weak battery. Either it is old and wearing out, or something caused it to become discharged.
This clicking sound can also occur due to bad battery connections at the terminals. They might appear perfectly clean, but sometimes a thin layer of oxidation develops between the battery post and wire terminal.
Always start with the simple fixes. Loosen and remove both battery terminals and clean them very well with sandpaper or terminal cleaner. If that doesn't solve your problem, either charge the battery or take it somewhere and have them load test it.
After that, there are too many things to start checking, but google will give you lots of ideas, depending how mechanically inclined you are.
I can't tell you how many times I have jumped to a diagnostic conclusion (i.e, mechanic friend "told me"), only to discover I over looked an obvious simple fix.
Best of luck to you.