Learn English – “accidentally” – Is “accidently” also acceptable

"Thru" is correct (however very informal, not a very good idea, and only used when space is at a real premium — e.g. road signs, technical drawings) English, but -u is not a shortened way of -ough except in words that derivate from through (e.g. breakthrough).

From memory, I can recall although, enough etc. where -ough can't be replaced by -u (althu, enu etc.), since in those words -ough doesn't have a /u/ sound. (However, although can be shortened to altho, as noted in a comment — however, Wiktionary and other dictionaries register it, noting that it's quite informal.)

You are confusing "Sever" with "Severe"

Severe is definitely used as an adjective. It means:

very great; intense.

While, sever is a verb which means:

divide by cutting or slicing, especially suddenly and forcibly.

put an end to (a connection or relationship); break off.

In your quote, the word "sever" is used in the Past Participle form, which is "sever+ed"