Hi Malcom,
That's a lot. I wasn't asking for advice and advice doesn't usually go well when you start off by saying "I haven't read everything". That said, your comments are fair, given your generalization of the situation.
This table is for my companies office. If there are any issues, I'll be able to fix them, and no one will be upset—That's why it's also fine that it's going slowly.
I believe this particular issue is straightforward enough with a clear solution. The steel is a known quantity, but the calculations on the load would be complicated because it's unclear how much load I need to transfer from the glue/domino joints to the under-frame. Additionally, I'm not counting just on the rigidity of the steel but the transfer of load to the other adjacent planks on the table through short stretch of steel. Finally, a pre-fabricated base supports most of the table fully, so the primary issue is about 18" on each end (as this was originally intended to be an 8' table). I also am accounting for additional weight, such as a person sitting on the edge or other forces, such as someone falling or dropping something onto the edge. I've used a structural engineer when I thought it necessary and this is not that case.
Derek