I see 5 questions here. We take only 1 per post

There's 1 question "can we hit it", any of the other 4 are ways to say "no, we can't, because…" — still, I'll edit to make that clearer.

@BenRW Every question mark is a question. Even between title and body. So if you post the exactly identical question in the title, and repeat it in the body, you're breaking the rules. This is a zero-nuance website. Please act accordingly.

@JohnO Noted and changed.

I'm thinking off the top of my head and don't believe my "answer" is ready for prime time. (a) It's really hard to turn in space, especially if you're moving in one direction really fast. That means that to "hit" the ship we simply need to get the missile in front of and in line with it. (b) 4.31mS is only 3-4 times faster than your average artillery shell. We have armor-piercing shells that don't travel through the (e.g.) tank. Therefore... (c) it appears on my napkin that it wouldn't be that hard to "hit" the ships.

... It seems to me that you're not really looking for a world rule but a story explanation (narrative necessity) to achieve a specific circumstantial condition. The movie Independence Day used the argument "and risk one falling object becoming many" as the reason they couldn't shoot down the ships. (Actually "wouldn't," they wanted to save invulnerability until later.) You just need to pick an explanation - it doesn't have to have anything to do with science or the rules of your world. It's a story rule (we don't do story rules).

@JohnO We're not quite that anal. Yes, we count question marks. But if the OP asks the same question in the title and in the body of the post, that's just one question. If someone's beaten you up in the past over asking the identical question in title & body and claiming that was two questions, give me the links and I'll remind them.

Those are some patient aliens! A lightyear is 9.5 TRILLION km so if the aliens are from a star 10ly away, it would have taken them 30 thousand years to get here.

"Realistically at least one country will want to destroy them": you say "close to present day", but how close? Realistically, only the US has the launch capacity and flexibility to even attempt the launch portion of such such a response today, and that's only because one particular company needed that capacity for their own plans. How much have space capabilities advanced? Are multiple countries sending people to Mars, or is the US still planning a return to the moon, with the main concern being maintaining a supply of pork to legacy contractors?

@SurpriseDog Unless the question was retroactively edited after you commented, it says 10 AU.

10 AU is where the aliens were detected by puny earth telescopes, not where their home solar system is.

@JohnO --- Actually, that is not true. Querents can, and are encouraged to, "repeat" the marquee question in the body. This is their opportunity to focus the attention grabbing title into a good worldbuilding question. That's not asking two different questions.

Also, your question is ambiguous about what exactly it is that "destroys the plot". Are you trying to ensure the alien craft survive, or are you trying to make sure that nobody even tries to attack them, no matter how ineffectual their attempt turns out to be?

10 AU in 6 months should start them at about 200km/s, unless they can boost much harder than 10 AU per 6 months per 6 months (because they'll spend most of the journey at 100km/s). But if they can boost much harder than that, the only good reason for them to be traveling so slowly across such a long distance is because they're afraid of hitting some tiny Kuiper belt pebble and dying, which means they can't be very hard to kill, either.

At some point, they'll have to start decelerating, so that'll slow them down. Unless they have inertial dampeners. Do they have inertial dampeners?

And they certainly won't be going 100 km/s in the Van Allen Belt, unless you want them to just whiz on by.

Whether or not all of Earth's countries can collectively have the discipline to not shoot the spaceshit can actually make for an extremely interesting subplot. You can put some suspense on that, put some pressure, do not tell the reader outright whether we'll manage to keep our fingers off the launch button. I refer you to Christopher Nolan's The Dark Knight, with the scene with the two boats with detonators; and also to Liu Cixin's The Dark Forest, a science-fiction novel that contains a big discussion about what Liu Cixin calls "cosmic sociology".