I've spent some time with maps and bus/train schedules of various vintages.
I have attached a couple of maps that are as large as the forum rules permit, at as high a resolution as possible.
They are Map # 2, and Map # 3. (Map #1 was too big to upload.)
They are scanned from Wanderkarte # 234T Willlisau, 1:50,000. 2009 edition. I marked them up some.
Die Wanderwege are shown as red lines.
Map #2 shows the region from Affoltern to Sumiswald and your possible hike route along a Wanderweg, as you described. Some other Wanderwege are shown, too. I have not been on that route, but have considered walking it. The marks on Map # 3 clarify where I have walked. I have been on the road and upper trails from point # 3 toward point #5 and beyond, as well as down the road at point #1.
Red dots along the road/rail line show bus stops. The Wanderkarten all show bus stops that way.
For fine detail, you have to go to the Swisstopo map that I linked you to before. A 1:50,000 map cannot show as much detail as that SwissTopo map. For example, the two-house village of Wydachen (not Wyclachen) only shows on the Swisstopo map, but I have put a red dot on the scans to show where it is.
Also, you then can discern Hegenegg, Oberhegen and Unterhegen.
Map #3 shows more detail than map #2 and has some numbers and other marks.
Attached image "Balloon..." was taken from location # 3, looking over Sumiswald
Attached Trachselwald Panorama was taken from the spot marked "Panorama" exactly at the red dot close to the "G" in "Gammenthal." It is just at the point where the road emerges from the woods.
The dark arrow #1 shows a scenic but rather steep section of road overlooking Sumiswald and Trachselwald in the distance.
#2 marks a schoolhouse called the Lueg Schulhaus. It has large windows with a view quite similar to the panorama photo. The children have great views, and the design shows how much the Swiss appreciate their Alpine and Emmental landscape.
#4 marks Schloss Trachselwald
#5 marks a ridge road that is a continuation of the road from Affoltern, also with nice views.
Here is the dilemma.
Sometime after 2002, the rail service north of Sumiswald to Affoltern Weier was discontinued, and the bus service put in place from Sumiswald to Affoltern-Weir train station, thence to Affoltern and on to Huttwil. The tracks are still there, used for freight trains.
The other bus line near the Schaukäserei goes from Affoltern to Hasle-Rüegsau station, to the south and west., along the yellow road due west of the Schaukäserei. You would not have crossed it if you went south from Affoltern.. The bus stops a few yards down the road from the Schaukäserei, and continues onward toward Haüsernmoos and Dürrenroth and Huttwil after coming up from Rüegsau. (I may not have the exact routes precisely correct around Affoltern, since they changed them a bit once the rail service to Affoltern Weir was eliminated.)
Study of the contours on maps 2 and 3 shows that. if you walked the Wanderweg that you proposed, and then came eastward to the bus route between Sumiswald and Weir, you would have made a steep descent of 136 meters from 816 to 680 meters, more or less. Gammenthal is in a valley. If you contiinued along the bus route to the south , you would then have had a climb back up to Sumiswald (elevation 700 meters), along a rather narrow and twisty road, then down again to the station Sumiswald-Grünen at 662 meters.
If you did not follow the bus route, you could have stayed in the valley, at nominal 660-680 meters, walked around the hill where the train tracks enter a tunnel, and worked your way to Sumiswald- Grünen Station via Oberfürten.
It is possible, depending on when you made the walk, that you caught a train at Affoltern Weir. Then, you would have headed East before point #3 marked on the map (perhaps at Wyden or Bühlfeld) , and walked the bus route northwards. You could only have caught a train from Affoltern-Weir before they discontinued train service ( between 2002 and 2009 sometime.) But, then , there was probably (not certainly) no bus service along that stretch of road south of Weier, since the trains were still running.
So, none of the options fit exactly with your description. The best fit is with a walk to Sumiswald-Grünen train station. However, if you recall the steep descent, or whether or not you went through Sumiswald, that helps understand what you did.
If, indeed , you go there again. please arrange for perfect weather and take in the views from above Gammenthal toward Trachselwald and Sumiswald.
That is as far as I can get with what I have from you, plus my knowledge of the region and the Wanderweg maps.
After I think about it some more, or have other comments from you, perhaps the route will become clearer.