@Paris Geek -- yes, the "most recently used folder" is not persistent, in fact, it's lost every time the app's process is killed.
I just didn't bother since I didn't know how it would turn out (and be received). Now that it looks like it turned out fairly well, I'll make a note to properly persist the chosen folder.
About third icon - not difficult technically, but not sure how it'll work...
I can only use about half the screen width for total swipe distance (nav drawer on the left, plus some slack so that swiping doesn't turn into a chore)... this means about 160 dp, or 50 dp per icon.
I'll make a note to try it out and see if it can work.