The multiple questions in the body are just ideas I have been mulling over, but not questions in themselves; the main question is in the body, so please your answer reflect just that.

I guess you mean that the main question is in the title? Then my answer is, in your alignment you know it.

More than that, you know then that the only partner you need is you, your Inner Being, your desire. When you're in alignment with You, you don't feel (and don't have) any need of a physical presence of a physical partner - and then the physical reality just solves itself out, in the best feeling way for you.

How do I know if I am unhappy about my relationship because of my partner or because of my thoughts about my partner?

You're always unhappy because of your thoughts - your unhappiness is your Emotional Guidance System at work, showing you that your current thoughts are not aligned with your desire, are not serving for you.

and you think your partner is the weakest link

That thought can feel bad to you - because you give in it your power away, and it can feel good - because it would remove from you your self-blaming. So in different states you can see how it makes you feel, to see if it is usable/serving you at that state.

Is it a matter of thinking differently about the relationship and the partner, and trying to get that better-feeling perspective?

It always is, because for YOU it's good to feel good, at all times, and even to leave the relationship better in a point, where you're aligned. (Better in the meaning that you will not draw with you what you "left behind" physically, you will go in a better feeling way, with life that would reflect that to you. It can give you more work on the way, too, but it may feel differently.)

The question is, will it be easier to you to change your vibration when you left the relationship, or when you're still in it. I believe there's no right or wrong - when you leave not resolving it inside, you take it with you, so you must see yourself, where it's easier for you to realign - to feel better, you will want to do it anyway.

If it is, won't there be a point when you actually need to let the relationship and partner go? How can that point be defined?

When you're aligned, you know it.

the idea of trying to think differently about my current relationship to try and improve it

That's why you're confused: you can try and think differently about your relationship for YOU, for feeling better, for coming to a better feeling place, not to try and improve the relationship.

Some healers say, that when they heal a person, that person can get healthy or make a transition - depending on how the alignment will express for that person, what for them would be a better solution, in their life.

Same about relationships - sometimes aligning lets you feel, that you grew out of this relationship, that it's not for you anymore. Then in a good feeling, you let it go - because you feel your energy differently, you know yourself differently than the energy of that relationship, do you see what I mean?

I think you can think the best thoughts about your partner, but when you're together, and you look at them, you just don't see the perspective you chose in them.

When you change your vibration, it expresses through your partner, too - or better said, through YOUR feelings when you see your partner. There you should look for the difference.

Bashar said it really well: "So it's not the circumstances in your life. It's how you respond to them that makes the difference in your life. It's not whether the outside world has changed in response to your spiritual growth that matters, it's whether your spiritual growth allows you to respond differently to the circumstances even if they stay the same. That means you have actually grown. Do not rely on the outer world as your measuring stick for your own spiritual growth, rely on your response to the outer world to determine how much you have grown. If you still react the same way to an old circumstance, you haven't grown. It has nothing to do with whether or not that circumstance should be in your life anymore."

From my experience with vibrational work, when I change my perspective, truly shift it, it reflects in the way I see the others, always.

I wonder if in the use of the law of attraction, one might find themselves doing things outside of what's conventional, e.g. possibly even considering polygamous relationships

Why not :) I saw this article I really loved, about the relationships, it speaks a lot about it: Fourth-Density Relationships. Precious precious article :)

Abraham-Hicks have this course, Sexuality - people ask Abraham there, if that or this is the right way to have a relationship, and Abraham says, that we only try to find what's "right" because we don't believe in our power to create our own lives, so we try to find an authority that gives it to us, that we would "behave" in accordance to what it would say. It's a powerful thought, isn't it?

constant attempts to fit a vibrational rationalisation into our day to day physical reality is exhausting

but interesting :)

can one just go with the flow, work on their vibration moment-to-moment, not ask such confusing questions and what will be will be?

I believe this is the way for me. Even though I love the questions, too. :)