What column header will reflect Predecessor Leads/Lags similar to MS Project?

"# of Predecessors"

Shows a link icon instead of text header though. 

You can add a field called Predecessor and then set the Format to show the name instead of the ID.

So the answer is - there isn't one.  

It appears they went through the effort to develop an column titled ''# of predecessors', then an icon to click, then a pop-up to click, to enter the lag.

Could we just have a "Lag" column with a numeric value?

That would be difficult since each line item can have multiple dependencies.

How was it solved for in Project?

Special coding that allows attribute modification on references to objects separated by commas. I haven't seen this done on an online solution yet and this is something we are not planning to support in the near future.