I get that some spells are "especially swift" to cast and thus have a casting time of "Bonus Action". But is there any reason I couldn't cast such a spell as my Action?
Does it make a difference if the idea is for the one spell to be cast all by itself on a turn or if the desire is to use another Bonus Action (possibly a spell) on the same turn?
No. Actions cannot be traded for bonus actions.
Various class features, spells and other abilities let you take an additional action on your turn called a bonus action.
You can take only one bonus action on your turn, so you must choose which bonus action to use when you have more than one.
You choose when to take your bonus action during your turn, unless the bonus action's timing is specified, and anything that deprives you of your ability to take actions also prevents you from taking a bonus action. (PBR, p. 69; PHB, p. 189)
So by the rules you could use one bonus action from a spell without using an actual action but you could not use one bonus action as the actual action and another one as the bonus action.
You get 1 bonus action and 1 action. The action cannot be traded for a bonus action.
Lead rules designer for 5e, Jeremy Crawford agrees in this tweet:
Q: Can a bonus action be used as an action? Using two bonus actions in one turn: one being the bonus action and the action to do a bonus action.
Crawford: Actions and bonus actions aren't interchangeable.
No, you can't cast a "bonus action" spell as an action. PHB page 202, under "Casting Time" (emphasis mine):
Bonus Action
A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided you haven't already taken a bonus action this turn. […]
It also addresses your question about casting a second spell in the same turn:
[…] You can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action.
So if you want to cast a bonus action spell (like Shillelagh) you must use your bonus action, but that could be before your regular action since the Shillelagh bonus action isn't dependent on another trigger. Then you could use your action to also cast a cantrip. Searing Smite, another bonus action spell, seems also designed to be cast before your action (which is then used to attack someone with a melee weapon).
About the only "double spell casting" in a round I can think of would be using your bonus action to cast something like Shillelegh on your turn, take an action (which could be a cantrip) also on your turn, and then use a Reaction to cast Shield during another combatant's turn.
No, you can't. Actions cannot be "traded" for bonus actions.
This is a rather weird issue since one would think that it should be possible based on how you'd envision it, but the RAW don't allow it.
The Player's Handbook puts the following limitation on the number of bonus actions (emphasis mine):
Your Turn - Bonus Actions
You can take only one bonus action on your turn, so you must choose which bonus action to use when you have more than one available. (PHB, p. 189)
However, spells with a casting time of one bonus action are described as follows (emphasis mine):
Casting Time - Bonus Action
A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven't already taken a bonus action this turn. You can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action. (PHB, p. 202)
Now, since spells with a casting time of a bonus action are described as "especially swift", one would think that using a regular action would suffice just as well. This view is further supported by the Sorcerer's Metamagic feature, Quickened Spell:
Metamagic - Quickened Spell
When you cast a spell that has a casting time of 1 action, you can spend 2 sorcery points to change the casting time to 1 bonus action for this casting. (PHB, p. 102)
Furthermore, there is a case to be made based on the wording of the casting time of one bonus action (emphasis, once more, mine):
You must use a bonus action on your turn to cast the spell, provided that you haven't already taken a bonus action this turn.
This can be taken to literally mean that you must use your bonus action only if you have not used it yet. However, the rules don't say what happens if you have already used your bonus action when you attempt to cast the spell. To me, the wording seems to imply that such a clause was supposed to be present, but it isn't.
So, while the rules technically don't allow for it, I don't see a reason, balance-wise, why you should not be able to use an action instead, provided that you have already used your bonus action for this turn and have yet to cast a spell. And I cannot think of a situation where you'd rather first spend your bonus action on something else in order to be able to cast a spell with a casting time of one bonus action with your action instead.
You can not do double bonus actions in 5th edition. You could in 4th edition if I'm not mistaken since the actions are related to an idea of effort and time. A round of combat takes about 6 seconds (10 rounds = 1 minute of combat). Bonus actions are so because they take either little effort or little time to perform so you could unsheathe a weapon (minor act.), attack (standard act.) and use some other minor action if you did not move in the same turn because you were "exchanging" actions. In this case, Standard - Move - Minor, became Standard - Minor - Minor.
However, in 5th edition things are a bit more static. One reaction, one bonus ,one standard and movement for each round of combat.
I houseruled this in my game so you can interchange like in 4th. For example, not move but take two bonus and one standard.
