This is a matter of conscience and one to take up with your pastor. There is no definition from the Church of what ‘counts’ regardless of what well-meaning people may post on this thread (i.e. ‘you have to be there for X’ or ‘OK if you get there before Y’).
The Church simply says you have an obligation and the obligation is to attend the mass. That means all of it, not part of it.
Now, as far as being excused from the obligation-- that goes hand in hand but is a separate issue. You are excused from the obligation when you have a serious reason. For example, flat tire on the way to mass, it’s the only mass, and you get there 20 minutes late. While you don’t meet the obligation of attending the mass, you also are excused from it due to a serious reason out of your control. I would see work the same way-- it’s the mass you can attend, you can’t be there when it starts, you do what you can. I would say your obligation is mitigated due to your situation.
And can I receive communion even if I miss the Readings?
Receiving the Eucharist is in no way tied to whether you heard or “missed” the readings. You can receive the Eucharist inside or outside of Mass. You can receive the Eucharist twice in one day as long as the second time is at a mass you have participated in (excepting viaticum).
So, whether you heard the readings or not is not relevant to the reception of the Eucharist.
On a related note, due to work I sometimes cannot find a Sunday Mass I can attend,
Then you are excused.
so I watch one live online and participate as much as I can (say the responses, etc).
That is a nice thing to do. It’s not in any way required, nor does it meet your obligation. but is good.
But if you cannot attend mass, you do not have an obligation.
Have I committed a mortal sin in this case by not attending mass physically?
What makes you think it would be? We are excused for just reasons.