Surprised nobody mentioned New Horizons.
I would suggest you just rough-form your ideas for the next few months until we have 1,000 time more information about Pluto than we have now.
I do wonder, if you try to land on a planet with a layer of frozen nitrogen, would you melt in and start to drown your ship in liquid nitrogen? Think of sticking a blowtorch on top of a glacier, pretty soon you’d have a blowtorch in a bubbling pool of water in the ice.
There’s also IIRC some story by Larry Niven, where Pluto has condensed in layers - a ship trying to land melts the oxygen layer into the hydrogen layer, setting the whole planet alight in a chain reaction.
World of Ptavvs. And of course once all the fire burns out then you’d have all the water ice you wanted a little later.
Your previous paragraph - that’s also Larry Niven, “Wait It Out” (a short story). The landing ship doesn’t melt enough ground to immerse itself but it does dig itself deep enough that they burn the engine out attempting take-off (over-simplifying).
To the OP, if you want to write hard science fiction then you might go the route Niven, Pournelle and some others adopted: earn a science-based degree first. If I wanted to write a novel set in a small French town then asking for advice on the internet would be a poor substitute for going and living in a small French town for a few years.
You could use the sky crane landing method that the martian Curiosity rover used. Perhaps even a variation of the airbag method of the other 2 rover twins once you were low enough as gravity is much lower. You would only need to keep the craft stable, airbag pointed down.
Most of this sounds good - I particularly like the Imperial British starfleet idea. But going to Pluto for ice is a lot of bother for nothing. Practically every moon of Jupiter, Saturn, Uranus and Neptune is about as icy as Pluto, and there is rock and useful elements there as well. If you build your generations ships in orbit around one of these planets then you will have plenty of ice. The easiest source of ice might turn out to be Saturn’s Rings.
You aren’t going to need lots of shielding, anyway. Generation ships travel so slowly that they could get away with quite minimal shielding. It would be nice to carry a source of usable water with you, however, and ice is a good radiation shield too. Just don’t be surprised if it is all gone by the time you get there.
Plus, ships designed to insulate against a vacuum probably aren’t designed to insulate / generate heat against immersion in liquid nitrogen, let alone liquid helium or liquid hydrogen.
You’re already lifting megatons to Earth orbit to assemble your ships, and you’ve already got a massive industrial plant devoted to that, and to on-orbit assembly. Just include the shielding. You can lift the water from Earth, then put it into molds in orbit and let it freeze into some convenient interlocking piece shape, then assemble all the pieces into your shield. Compared to the other much more complex lifting and assembly you’re already doing to build the ship, building the shield from Earth water is a small addition.
They probably are NOT building the generation ship out of materials from Earth, unless they have a ridiculously cheap method of getting stuff out of Earth’s gravity well. Almost certainly it’s being built from materials mined from moons and asteroids. When O’neil and co. were proposing orbital colonies back in the 1970s, it was based on an analysis that beyond a certain point it would be cheaper to establish an industrial base in space than to keep shipping stuff from Earth.
If I wanted to write a novel set in a small French town then asking for advice on the internet would be a poor substitute for going and living in a small French town for a few years.