stut

Yes, and the whole station is interconnected, making something of a mockery of having some barriers closed while others are open...

However, yes, the barriers are left open for big chunks of the day.

Not that there's anything to actually see on platform 9 (nor is there a wall between platforms 9 and 10 - there are tracks separating them). However, the trolley-in-the-wall thing by the bookshop is very popular, and if you want a good view of platform 9, head up to the Parcel Yard pub (children are welcome, and it's a pleasant a quirky place for a coffee and/or bite to eat) which has windows overlooking them.

andreadbc

I haven't done a walking tour of film sites (went on the Jack the Ripper walk), but I went on the Studio Tour a few months ago. I would recommend the Studio Tour as it is a very detailed experience, loads of information and picture opportunities with different sets from the movies (even though I'm more of a fan of the books than of the movies, I really enjoyed it and I was there for about 3 hours)... The model of Hogwarts is a real standout.

GregWTravels

I have not done both, but I did the studio tour and my sisters did the walking tour.

I am not a Harry Potter fan - I haven't read the books and only saw the first film - however I quite enjoyed the studio tour. There was a lot about film making on the tour to keep me entertained in addition to all the Harry Potter stuff which kept my friend entertained. I really enjoyed seeing the sets, the props and the models.

My sisters did the walking tour. They said the guide they have was very, very good - entertaining and funny. I got the impression that the tour would probably be more interesting for those immersed in the film, as it is mostly just showing you sites where things were filmed, so I think recognition of the sites is important. But other than the King's Cross cart-in-the-wall, most of the sites are not "Harry Potter" specific, so just buildings otherwise. The Harry Potter content comes from knowing the films and whatever the guide tells you about the location.

big_mac

My sisters also enjoyed the studio tour, but haven't done the walking tour. I am sure that they also enjoyed the retail opportunities you get at the studio (and I have a packet of Bott's every flavour beans to prove it!).

There are currently no train services loaded into the TFL journey planner for after 17th May; it's nothing to do with possible line closures.

There is just about no chance of them moving the engineering works to a midweek date in the future. As for any transport, things could still go wrong, but it's not that common and the overground route is a reasonable alternative.

King's Cross is a 10-15 minute walk from Euston, it's not a major detour. I think the photo opportunity at platform 9 3/4 is a little underwhelming, but it does seem quite popular.

My guess is that you won't actually need to decide until much closer to the date; most schools won't be on holiday so those dates shouldn't be very heavily demanded.
I would worry more about the possibility of bad weather which could make any walking tour a bit miserable.