Need help with Heroic Intervention and DC Dreadnought ability (noob question)

I don't understand clearly how Heroic Intervention Stratagem is supposed to work.

1- First, let's assume you charge successfully with a Death Company Dreadnought and get within engagement range of an enemy unit.
2- Other enemy unit use Heroic Intervention and charge you successfully, so you are now with 2 different units within engagement range.
3-Not sure what happened next. It's just after your charge, so I suppose that if nobody has "Fight First", the enemy unit that charged you attack first, because it's not its turn. And you fight it back because Frenzied Reprisal ability (automatically).

4- What happens now? Previous step is "out-of-phase", so the "real" fight phase starts here, right? So I can choose to fight with my Dreadnought whichever I want of my 2 units next to me. But... what if I don't have a DC Dreadnought nor Frenzied Reprisal... Can I actually attack?, because rules says that a unit can only fight once in fight phase and I could fight back heroic intervention.

Sorry if weird question, but appreciate clarifications about the the fight interactions between my attacker, the defender, and the unit that interrrupt with heroic intervention. Thank you so much!