I know the halacha is mezuzot need to be checked twice every seven years. Does this mean every 3.5 years, or is it more of a loose requirement?
Also, does this apply to mezuzot that are indoors? Nowadays when mezuzot indoors and are wrapped in plastic and also in a protective case, I find it hard to imagine how they could become cracked or damaged.
You err in your first statement and are right on target in your second.
Meaning indoor mezuzot DO NOT need to be checked at all, unless you have reason to suspect that they are damaged, in which case you better not wait 3 years either 
As to the outdoor ones, it’s very important to take appropriate measures in ensure that it’s watertight and/or not in the broiling sun etc. and if you feel it may be a problem, by all means check them
Does the rav have an opinion on whether to place a mezuzah straight (according to rashi) or on a slant?
The Rav places his mezuzot on a slant.