Entering music using the computer keyboard only

I think you must re-read the Hand Book, tie works perfectly, slurs also, you surely make mistake

You wrote:
when trying to create a tie between two notes, the second one gets replaced with a copy of the first...

As you are entering notes, and before you press the + key (which creates the tie), enter the duration for the tied note. Then press +.
Without selecting the (different) duration, a copy of the first note is made.

See:
https://musescore.org/en/handbook/ties#add-ties-note-input

You wrote: "I can't find any shortcuts for common symbols like dynamics, articulations, trills, etc. "

The handbook (https://musescore.org/en/handbook/articulations-and-ornaments and https://musescore.org/en/handbook/keyboard-shortcuts#articulations) documents the shortcuts for staccato, tenuto, sforzato (accent), marcato and acciacatura. It is possible to assign your own shortcuts for other items via Edit->Preferences->Shortcuts (see here https://musescore.org/en/handbook/preferences#shortcuts). Or you can add an articulation or ornament from a palette with a single click without leaving note entry mode. It is a nuisance to have to resort to the mouse, but if you get the shortcuts set up for the most frequent articulations and ornaments in your use case, then you won't have to use the mouse very often.

Unfortunately There is currently no shortcut yet to add a dynamic, but you can just click on the desired dynamic in the palette without leaving note entry mode. But I believe that addition of dynamics via shortcuts is is being considered/under development. I can understand that there are many possible dynamics and only so many keys on a keyboard so the user interface needs to be carefully thought out.

You also wrote "how do I create grey or dashed symbols (for editorial marks)?"

You can change the colour of items via the Inspector. But see also #300462: Add square brackets to score elements to indicate editorial additions/changes where editorial markings are discussed.

Ties definitely work, as mentioned, you should be entering the tie immediately after the first note, there should be no second note, this gets entered automatically. So it's more efficient than you were probably thinking. Also, not sure what you mean about "losing your place", if you mean you just forget where you are, I can't help with that specifically, but I can point out when you leave note input mode the same note or rest that was selected before remains selected. And if you ever lose your selection (for example by pressing Esc), Shift+L will re-establish a selection on the most recently selected note. Also Alt+Left/Right never lose track of the selection ever, and these are extremely useful commands for keyboard users since they also let you move to focus to things other than notes or rests.

Many palette symbols have shortcuts, as mentioned (eg, staccato, sforzato, marcato, hairpins, slurs, most forms of text, etc), see the Handbook for more info. Also see Edit / Preferences / Shortcuts where you can define your own shortcuts for additional commands. But it's true that many palette elements - including dynamics - don't have direct shortcuts. Still, that doesn't mean you need the mouse. Ctrl+F9 does a palette search, you can type the name of a symbol and use the arrow keys to navigate the results, and apply the selected palette element with Enter. You can customize the arrangement of the palette however you like to optimize it for the symbols you use most.

We've actually gone to considerable trouble to make MuseScore as keyboard-friendly as possible, because we want it to be fully accessible for blind users. See the Accessibility section of the Handbook for more on the various shortcuts and ways of operating with keyboard only,