Scenario one: I'm not sure anything would happen. At the moment the Republican party is joined a the hip. No Republican politician can win a primary without attaching himself to Trump. This would mean that voting for impeachment regardless of evidence is political suicide.
Scenario two: Pretty unlikely. What is available in the public realm is more then enough to conclude that Trump will never be completely vindicated. Even if it's shown that Trump didn't coordinate with Russia directly. We know the people in his campaign tried on his behalf and we know that Trump was/is actively trying to hamper the investigation. Something that could very well be construed as being a crime in itself.
Not to mention this new charge of campaign finance violations.
As to what would happen, well if I'm not convinced him being guilty would prompt the Republican party then him being as clean as this wouldn't either.
Scenario three: I don't think stupidity is a valid defense in criminal cases, nor do I think guilty pleas and sometimes pretty lengthy prison sentences can be described as dishonesty. I get what you are saying but it seems a bit light.
- Anyways a president can be charged and convicted, for sure after he leaves office. Probably while still in office. The United States v Nixon established that the president does have to comply with a subpoena for instance, something that suggests that he can be charged too, although this would most certainly go before the supreme court. The most likely outcome would be that impeachment procedures would follow, procedures by the way that don't need an actual crime to be successful
-As a side note most European countries have similar procedures of removing people from office. Berlusconi was forced to resign for instance. Willy Claes in Belgium
Willy Claes - Wikipedia
John Profumo in the UK
Profumo affair - Wikipedia