Indeed, the timetable can be confusing, because it contains a large amount of information in multiple layers of menus.
Here is a start on your questions.
#1- Grindelwald to Jungfrau (please note correct spellings)
When I open the timetable and list Jungfrau as a destination, I get a long list of stations to choose from.
The first one on the list, even though I entered "Jungfrau" is "Jungfraujoch." That is the name of the station and the ridge or pass where the train line ends.
From Google Translate seeking the English translation and German synonyms for "das Joch " I get-
<<"Translations of Joch
yoke - Joch, Passe, Gespann, Tragjoch, Schultertrage
ridge - Grat, Kamm, Rippe, First, Rücken, Joch">>
So, to be precise, your destination is the saddle or pass on the Jungfrau.
Always, the first entry in the list in the timetable is the simplest and the main location or station. When in doubt, pick the first one. (By the way, I did not get a listing for the word "Jungfrau" because there is no station named Jungfrau).
Other names may be nearby bus stations or cableway stations. Especially in cities, you get a long list of bus and tram stops.
Often, the second listing is a name, followed by the word "station" in German and the equivalent la "gare" in French. That is the bus stop, which might be just around the building, across the street, or 2-3 minutes walk from where the trains stop.
Note in the attached screen grab , where I opened the details for the first train by clicking on the plus to the left of the listing, that a map is available in the footnotes. That map shows the journey in detail, and in the menu choices, lets you see an expanded map of the area near the start and finish of the journey.
<<"The timetable is showing only 3 trains from Interlaken to Jungfrau (i plan to board it at Grinderwald). This possibly cannot be correct.">>
Indeed, it is incorrect in several ways.
First, you did not spell Grindelwald correctly. Second, a look at this map will show you that the route from Interlaken to Grindelwald can go through Lauterbrunnen or through Grindelwald.
You should not (as you have stated) got to the Jungfrau via Interlaken boarding at Grindelwald, unless you wish to take a less direct route, for example, to your favorite coffee shop in Interlaken before you go to the Jungfrau. ;-)
Follow this link and zoom in.
Once you have zoomed in far enough, you will see the train lines.
(If you enter the menus for Points of Interest/Traffic, you can turn on an overlay of stations for different types of transport).
To get more listings, you can click on the "later" and "earlier" links at the bottom of the timetable. default shows only 4 journeys.
As explained in the link you have mentioned in your post, for the timetable, to force the journey through a particular station, you should enter the name of the station in the "connections "(or, "via") box.
You will enter Interlaken as your start, Jungfrau(joch) as your destination, and Grindelwald as your connection or via point.
That will give you what you seek.
<<"Or what is the best way to travel from Milan airport to Interlaken.">>
this requires a long answer, and at this moment, I don't have time to pick out the details.
In general, there is bus service to Chiasso, or, train/bus connections to the line betwen Milano Centrale and Domodossola.
The Swiss timetable will show you the way. See attached screen grab.
Let us know if you need more details.