Sorry, Wayne, but I know very little about postal history and covers, as I primarily collect "off-paper" postage stamps. In addition, I cannot read Chinese with understanding, although I can recognize some of the characters. Since you are in Taiwan, I'm assuming you can read Chinese far better than I can.
My best reply/guess to your questions are...
-- The green label affixed to the back of the envelope is an express delivery registration tag. I don't know if it could also be used as a simple registered mail tag because the wording next to 0618 is a bit ambiguous; but I assume in this case it was used for express delivery, rather than registered mail.
-- If express delivery, then there should be a red special delivery stamp in front. I believe it was there, but over the years, either fell off or was removed. It is not unusual for stamps to fall off covers from that era. I do have some covers and letter sheets from the 1930s and 1940s, and it wasn't unusual to have a few stamps fall off.
-- The cover was sent to Miss Liu (at least it looks like "Liu" to me).
-- I don't collect or study postmarks either. Most of my China collection from that era is archived, so this is from memory. I recall 2 basic postmark formats for the date. Looking at the columns in reverse order (i.e., from left to right), the 2 most common formats I've seen are:
1st column: I believe that is the hour in military time format.
1 vertical bar
2nd column: date.
3rd column: month number.
4th/5th column: sometimes there are 2 columns, sometimes only 1. Your example is the 5 column type. I don't know what the 4th column is for. Your 4th column is definitely not the year inscription. However, the rightmost column will be 3 characters corresponding to 2 digits and the word "year". The 2 digit year will be relative to the year 1911 (i.e., add that number to 1911). The red oval I drew is where that year column should be, and I cannot make out what it says.
The most I can make out is what you already stated, sent Apr 21 and receiving post office date of Apr 22.
That's all I can tell you beyond what you already knew. There are a couple other SCF members who are fluent in Chinese. Hopefully, they can chime in and accurately tell you more.
Hope that helps.

k