Ranty answer, to try and get the "mindset" of someone out there why bounty questions get less attention.
I took a quick look at the linked questions.
My quick reactions:
The first one: iosswift - I don't know those languages. Question looks hard.
The third one: vba - Bleh. I don't like VBA either; It has funny syntax and it's something that serious yet I have no idea what you'd use it for.
The fourth one: vba - More bleh.
You can add bonuses to questions I don't like all you want, it's not gonna help the part where those questions don't interest me.
Now, the recent hot questions...
Celebratory fireworks animation
Meta-effect. It's about a meta event (CR graduation).
Regularity in the "Rusty Towel of Mutual understanding"
Java. I like Java. Also, the question was fun.
Proof that I was wrong about Random Number Generators
C#. Follow-up... and... meta?
The Term-inator: Pi edition
C++. Seems to have a fun title.
If she floats then she is not a witch like we thought
C++, fun title.
Adding a course to one of 8 periods
Java. Interesting title.
In an all-you-can-eat restaurant, you cannot tell me to "eat your greens, you'll get dessert after that" because I can get up and fetch dessert myself.
On places like Stack Overflow, reputation is "hard" to get. (Kinda?) So when someone offers you a huge chunk of reputation, that's worth a lot.
Here? 50 points?
Most questions around here, you post an answer, you will get 20-30 points. Possibly 50-80 points. If you get 90 points (9 upvotes) you'll get another upvote from someone via the 1-short of a badge SEDE query. And those results are cached so you might get two upvotes.
There's plenty of questions to answer. There's many answers for a single question, so you need less overall knowledge to have an answer to a question.
With a 50-point bounty, all you're doing is adding a small bonus. A small bonus in points that are plenty. I don't care about that bonus. I give those questions a look from time to time, but generally they come with a ton of baggage: The question makes me spend a lot of effort to answer it. And I don't even like the question. So I'm not gonna answer it.
Then there's the whole thing surrounding bounties: Only one person gets the bounty. The rest gets nothing.
If upvotes gave +1 and accepted answers still gave +15 Code Review would be a mighty different place when it came to points.
I have 20 accepted answers. That makes for 300 reputation. I have accounts of other sites for +100. I once asked a question and got 11 upvotes (55 reputation). I made a couple edits before I got to 2K. Maybe 500 reputation of my 4.1k is non-upvotes. The rest is all upvotes.
It's far easier to get upvotes than accepted answers. 360 upvotes versus 20 accepted answers...
End rant, lets look at your sub-questions.
Are my observations correct? Can anyone back them up with SEDE data about the usefulness of bounties? Possibly compare the upvotes/views/answers of the question before the bounty started with the situation once the bounty has ended?
I don't have SEDE for you, but I feel bounties are useless. You can apply them once your question has started to fall into zombie territory. All you're doing is making it a special zombie. It's not gonna get your question answered like it would on SO.
Is it really a problem that questions with a bounty do not get more attention?
No? Yes? It's sad if you ask a question and it becomes a zombie. Not being able to get people to answer it... yeah, that's sad.
But TBH, the reason I'm not answering your question is because it's not interesting to me or because I don't have the knowledge.
You've either posted your obscure thingmajig that does something that's probably useful to someone (but not to me). In this case, you want a full review of your thingmajig; I guess it does what it needs to do and all we can do at this point is be an expert of the subject of thingmajiging to realize your thingmajig needs some extra helper functionality to be thingmajig-A3-B compliant. Or maybe find some nitpicks that you're happy to know, but weren't really interested in. Basically you've made something and are verifying its quality by a lack of answers. Most of us are not gonna write "Looks like you've done it all correctly" answers because those can be wrong. Maybe I would write an answer like that, but you state that you don't like the way you did X or Y in your thingmajig. And I agree, I don't like that either. I don't have a solution for you, so I can't tell you you did everything right either.
Maybe you have a design flaw that you'd want to get fixed ("I have a problem, please fix it in this review"). And since nobody answered it you added a bounty. I, perhaps incorrectly, assume said bounty requires me to solve your problem. I can't do that for you, thus I don't answer your question.
In these cases more attention is given, but none of this attention results in answers. Because we don't know. Or because your question looks like we won't know. Whether that's a problem... well... yeah, it is, if you relied on CR to get things done for you. I hope not.
Should the system be changed StackExchange-wide? For example, give the featured questions a more prominent place on the homepage, or let it appear in a bar similar to the hot network questions?
Hot questions are hot. Bounties are not. Bounties are difficult questions that you can answer for big points. Usually hot questions are questions that can be easily answered. They get answers for various angles. On a hot question, you get one style review, one algorithm optimization, a general optimization, a sidecase review, error handling and maybe some alternative solutions. And they all cover something different.
On bounty questions those things aren't available. You got your whole system already and it needs to do this cleaner or better or what have you. Usually, this means one answer is correct. The rest are nice to have. Also, the question is harder.
Questions go hot and get bounties for different, mostly even opposing reasons.
Hot questions are mostly easy. Everybody understands what's going on and everybody understands the solutions. That's why it's hot; it's understandable and relatable.
Bounty questions are mostly hard. Someone wants it to get answered but it hasn't yet. It's deep and only for those with the knowledge. It's a far more localized issue. That's not likely to go hot.