What I see, is a split (6/7/7) is nonsense ending, then 20, 20, and 20 are endings 2-4. Then a split (6/7/7) after getting the first 4 endings results in ending 5(harem).
Of course. Reading My Dear Waifu has 20 chapters total, regardless of the choices you make. There are 5 endings. Ending #1 is if you split your choices between the 3 options, like picking option one 6 times, option two 7 times, and option three 7 times. Endings #2, #3, and #4 are if you pick each of the options 20 times total. And finally Ending #5 is doing the split a second time AFTER already unlocking all 4 of the other endings. You need to read the book start to finish 5 times to unlock the voucher.