about the missing value problem. You can use the Missing Value node to treat your missing values, e.g. you could delete all rows including a missing value. But what I would do first is to go through the workflow and check which node creates the missing data and why.
About question b: I suppose the checkbox your are talking about is "Allow changing table specifications" in the loop end node, correct? If yes, I suppose that you have different column names or a different number of columns in each iteration. In this case you have to activate this checkbox to make the loop running.
About question a: Could you share a screenshot of the configuration window of the Excel Appender node and maybe part of the table you want to write?
I do not mean, that there are missing values, but that alle the values that were present before the generation of the Excel Sheet are now (afterwards) missing.
The loop is running now after I checked the boxes for having different table structures from the beginning of the loop.
I attach a new Screenshot for the new structure of the workflow and a screenshot of the configuration of the Excel Sheet node.