Well tstats really needs to be the first command in the search so, what I would suggest to you is:
After the tstats command, use an eval host=lower(host), eval source=lower(source), and then redo the same calculation (which is now very light because you;ll have very few results, like this:
| tstats latest(_time) as latest,earliest(_time) as earliest WHERE index = * by host source
| eval host=lower(host), source=lower(source)
| stats latest(_time) as latest,earliest(_time) by host source
... View more