IF Joe had done the right thing a year ago and announced he would not seek reelection, and IF the Dem party had respected the competitive nomination process, do you believe Harris would have competed and won the nomination,

That's the most likely outcome. These things are never certain, of course, but nearly every Vice President that's sought their party's nomination for president has been successful in getting the nom. Nixon, Humphrey, Mondale, Bush, Gore, Biden. IIRC, the only VP that's sought the nomination that didn't get it was Quayle in 2000. The Vice Presidency may not be worth a bucket of warm piss as a job, but it's an incredibly strong position from which to run a campaign for the nomination - you have prominence, fundraising ability, access to every elected official in your party, and the ear of the President to serve as a carrot/cudgel for lining up support. In any nominating contest, the VP starts as the early leader, if s/he runs.

and if not, can you name one Dem who would have won the right to represent the party who wouldn't have beat trump, easy?

All of them. Every single one.

Harris' main problem isn't who she is. It's that voters are fairly unhappy with conditions in the country. Voters are livid about inflation and distraught about immigration and border policy. They blame Democrats for this. And Democratic voters are disappointed in how their "trifeca" control of the federal government (House, Senate, Presidency) during the first two years of the Biden Administration resulted in so few of their priorities getting enacted (hi, Joe Manchin!).

Most voters disliked the chaos and craziness of the Trump years, but they look back on a time of low inflation, low interest rates, low unemployment, steady (if not exceptional) GDP growth, and few foreign hot spots with positivity. The GOP has successfully raised the saliency of immigration and border control as an issue, and inflation was always a top voter concern. Every Democratic candidate would have run into the same major core issues that are giving Trump 46%+ of the vote.