I must have missed this earlier -- do all -ig adjectives go without the -t suffix in neuter? Never umuligt' / kjedeligt' / *riktigt? Do any other adjective types forgo the neuter ending?
There's a note about this in the lesson tips and notes: "Many adjectives ending in -ig or -sk, like viktig and norsk, do not sound pleasant with a -t ending. This is why we do not add a -t to these specific adjectives in the neuter form."
Is "u-" added to words to mean "not" just like how english adds "im-" to words like possible and impossible?
Yes, but the prefix u- can both be used both to express opposite meanings and to make a negative version of words:
uvanlig = unusal (the opposite of usual)
uvane = bad habit (not the opposite of a habit, but rather a negative version of it)
We also use the prefix a- for some words, much like in English:
atypisk = atypical
could this also mean "it is not possible" or would that have to be "det er ikke mulig"?