400 grit if fine as long as used lightly and rubbed together as instructed.
More to the point is what kind of tung oil is being used and how long was it left to dry.
The point of shellac on soft timber is exacty to prevent this happening,
I, think you may have sanded too hard in spots or the tung oil isnt drying.
You should only need about 3 coats max. and should let dry at least 24 hours between coats.
I did suggest cabots Danish Oil not tung oil, but unless the tung oil is pure
as opposed to a the usually commercial product, the effect should be much the same.
Dont worry, as ern says, let it dry for a week and then take another look.
Once its thouroughly dry, you should be able to sand it back and put another coat on.
And I have never seen a professional restorer, and i know a few, to use a sanding block, when cutting back a finish, you cant "feel"
this finish through a block.
Astrid