Newbie First Post Stupid Question

Hello, and welcome! The Handbook is a good source of information, but not the only one. Be sure also to check out the tutorials under the Support menu above. Also, there are a number of additional resources on my site https://masteringmusescore.com.

I'll try to address your specific questions. If you need further assistance, I encourage you to attach your score and describe what you are trying to do in more detail, so we can understand and assist better.

To change the duration of a note, simply click it and then change its duration using the duration toolbar. This changes that note only, leaving other notes exactly where you entered them. So if you change the first quarter to an eighth, that leaves the remaining quarters right where you entered them - on beats two, three, and four. This leaves a rest on the "and" of 1, where you can enter your second eighth.

Not sure what you mean about not being able to click on a measure and go into note input mode. It should be as simple as clicking anywhere in the measure and pressing "N". What goes wrong when you try? Seems you must have successfully done it at least once before, to enter those four quarter notes?

It is generally true that MuseScore expects you to enter things left to right for the most part You can certainly go back and change pitch or durations of notes, or move things around via cut and paste, but you're best off doing a fair amount of thinking before you start.

Meanwhile, here are a couple of resources from my site you might find useful.

Click below for my Quick Answer on how to enter notes:

Enter Notes

https://masteringmusescore.com/quick-answers/enter-notes

Click below for my Quick Answer on how to enter tablature:

Enter Tablature

https://masteringmusescore.com/quick-answers/enter-tablature

You wrote:
A lot of what I am doing is experimenting with beats and notes, and unless I input notes via keyboard in a stream of data, I can't easily go back and edit what I've done.

For experimenting with beats and notes - and instead of editing what you've done - simply write several measures at a time (each with different beats and notes). Then play them back and delete the one(s) you don't want.
For deleting measure(s), see:
https://musescore.org/en/handbook/measure-operations#delete

The reason I mention this is because editing notes within a measure can be tricky for a new user.
For example:
Deleting a note will leave a rest where the note was formerly located. (Maybe you don't want a rest there.)
Decreasing a note's duration will add a rest to fill up the time. (So a quarter note changed to an eighth duration will yield an eighth note followed by an eighth rest.)
Increasing a note's duration will overtake time from the following note(s) or rest(s).

On the other hand:
Deleting a selected measure (Cmd+Del on a Mac computer) will remove the measure and all its contents, filling the void with the music that follows.
This may be easier for you until you better understand how MuseScore works.

(Also, compare Cmd+Del to simply using the Del key. The Del key deletes the contents of the selected measure and will leave an empty measure behind.)

Regards, and welcome aboard.