gonna answer best i can one at a time and then clarify how i feel this works.
Does that mean...
The barrier & platform 9 & 3/4 is just a barrier (it's solid & wizards can't get through) --- it is just a brick wall now, no in and no out. however that only means people cant get to the platform from that side. the train can still leave the station. since school is still in session i believe it wouldn't matter much since the train doesn't operate during term. if they were out of session the train might still bring them home but have to stop somewhere down the line and then they would have to find another means of transportation to get back to their homes.
The portraits are just normal portraits? -- yes the portraits are just normal paintings now. they no longer control the entry ways to common rooms. i believe the staff at hogwarts would of just taken them down because they can no longer magically swing open and allow students in or out of the rooms. the passageways behind the portraits were never enchanted to seal (like the diagon alley wall) so they are just open archways now.
House elves can't Apparate? -- house elves are considered magically creatures right? if they are then that means their special breed of magic does still work. only wizards have been stripped of their magic and things that wizards have created no longer functions (magical potions, warding, charms that light fires, and so on.) creatures still have their magic so they are super extra dangerous now.
12 Grimmauld place is visible? -- that one is a good one.... if we go by the fact that "it was always there, but muggles never see anything" then yes it is there and a muggle postmen could technically get to the landing now. if we go by movie standards, where it kind of pushs itself out of no where then i have no idea.... help? lmao.
The leaky cauldron is visible? -- as far as i can remember the leaky cauldron was always visible but to muggles it never seemed like a good place to go. it was enchanted to discourage muggles from entering and look more like a run down abandoned pub. almost like a glamour to keep them away i think.
Floo powder doesn't work? --if we go by what i said earlier, floo powder is a wizard invention and will no longer work.
Diagon alley is accessible by muggles (is the wall now just a passageway or is the wall a solid wall and Diagon alley sealed off?) -- the leaky cauldron is accessible to muggles but i highly doubt the staff lets anyone not magical into the backroom and to the passage out back that leads to the alley. there was no definitive idea set out for this but in one of my threads i have it played out that the brick wall became a solid wall and the wizards knocked some of it down so the passage way still exists but never closes.
Elemental magic doesn't work? -- we didn't discuss this but since elemental is a natural thing, occurring without the influence of wizards and their magic i would kind of classify it as a special magic like beasts and house elves, so i think it might still function but id like more input on that?
