The IntelliGantt Web Part can show all the tasks in your hierarchy AND multiple predecessors. We had a couple users ask about this and I'm happy to reply unequivocally 'Yes'.
The IntelliGantt Web Part uses a SharePoint LookupMulti column, parses through the values, resolves the values to actual predecessor items and then computes the visible line drawing. It looks like this:
If your predecessor SharePoint column only holds a single value ('Lookup'), IntelliGantt handles that just fine.
Same thing for Assigned To and any other Lookup or LookupMulti columns in your SharePoint list.
Keep the questions coming, we're happy to answer them.