The mice showed up millions of years after the planet was ordered from Magrethea. I don't imagine they were checking up on its progress all that time.

Huh? The mice - or the hyper-intelligent pan-dimensional beings they represented, anyway - are the ones who ordered Earth to start with. Millions of years ago, from our perspective.

Even Slartibartfast said so.

We saw them as "humans" addressing Deep Thought of course, but in our dimension they appear to be mice.

As with all things HHGTTG, this is all very confusing. In the radio and TV shows, when Frankie and Benjy say "we ordered" the planet, they are using the royal "we." They mean their race ordered it. In the movie, they ARE the same characters that spoke to Deep Thought, but we're talking about the TV show here. Lunkwill and Fook received Deep Thought's answer and ordered the planet Earth from Magrethea (after arguing with the philosophers). Frankie and Benjy (the mice), as representatives of their pan-dimensional race, came to retrieve the Ultimate Answer from Arthur's brain since their planet was destroyed.

Oh, I see what you mean. Whether or not they were the same mice.

But Slartibartfast also said that the mice were "interacting" with Earth the whole time, with all the stuff about mice running down the maze the wrong way, etc.

Since mice were known on Earth for a very long time, seems likely that they were always the hyper-intelligent, pan-dimensional beings. Which means they would have known about the Golgafrinchans, unless you're like to argue that there were no mice on Earth at that time?

Okay, I see what you're saying. I suppose we're going to have to believe the mice didn't know or if they did know they didn't care. Honestly, their "interactions" probably messed with the program, too. 🤷🏻‍♂️

Yes, that's all possible. But mostly I didn't care for the pointlessness of it all, even though some claim that's supposed to be the point of it all: the pointlessness.

Apparently the Ultimate Question - even if they had gotten it right - was meaningless.

"What do you get if you multiply six by nine?" was obviously pointless.

But the CORRECT question (to get 42), "What do you get if you multiply six by seven?" is also pointless.

Nor would the Earth-computer need to be constructed to find it.

The various suggestions made when "42" was revealed, such as "How many roads must a man walk down?" really aren't any better.