I wish to use ppml to estimate a structural gravity model for the years 2000-2016, for ~ 130 countries.
I have some data constraints, and I would like to know if it is plausible to still use ppml under these circumstances.
Specifically, I wish to know:

1. Can one use the ppml estimator when the data is unbalanced?

2. Can one use the ppml estimator when there are gaps in the data? e.g., I have trade data between South Africa and Peru for the years 200-2003, 2007, 2010-2016, and trade data between Israel and Taiwan between 200-2005 and 2008-2016.

3. Can one use the ppml estimator when I have data for just one country in a pair? e.g., I have data on exports from the UK the Poland but not vice-versa.

Many thanks,
Dan