[RPG] Are actions and bonus actions interchangeable

The way bonus actions work is:

1. You can't take more than one bonus action per turn
2. If using an ability/spell/whatever is done as a bonus action, you can do it (bearing in mind #1)

So bonus actions aren't something you have or are granted — they are something you do up to a limit of one. Casting the spell is itself doing or taking the bonus action. Anything that is a bonus action means "you can do this on your turn, as a bonus, if you haven't already done some other bonus-type action."

D&D Basic Rules v0.1, page 69 (emphasis mine):

Various class features, spells, and other abilities let you take an additional action on your turn called a bonus action.

You never need something else to give you a bonus action before you can take a bonus action. It is only a description of the type of action it is when you do that thing. It's not a point or expendable that you collect first, and then spend. It's a type of action.

The converse is that if you don't have anything to do this turn that is a bonus action, then you can't take even one bonus action this turn.

So smoke em' if you've got em'!

You said it yourself, witch bolt "ends if you use your action to do anything else". Using your bonus action isn't using your action.

In 5e, it's just "action" not "standard" action, and as you also said "movement" actions aren't a thing. PH 189 says that "On your turn, you can move a distance up to your speed and take one action."

If you spend that one action doing not-witch bolt, the spell ends. Anything else you do with your move, bonus action, or reaction are immaterial—unless those actions violate one of the restrictions of the spell you already listed (like using your bonus action to misty step out of line of sight).