What is the "default" font for iPad mail program
It should be San Francisco, the system typeface in iOS, see Apple Typography guidelines

I tried to check comparing it to similar typefaces both in iPad mail and in TextEdit.

Here's mail.

Here's TextEdit.


I took "i"s and "a"s as a confirmation that the default is SF.
You can see that the length of the first lines are different, but SF has different tracking depending on the context.

And yes, it doesn't seem to be possible to choose again the default font in iPad mail...
I succeeded in getting it back by pasting the text somewhere else (i used the Notes app) and copy-pasting again in mail.