にわたって vs にかけて vs を通して

spans or extends across a period or range

にわたって (and its verb form わたる) expresses that something extends over a whole period or range, such as six years, several miles, or all ages and classes. It attaches to a quantity or scope word with に, as in ６年にわたって, and わたる can also follow に in patterns like みなにいきわたる.

1. ついこの間あいだまで、６年ねんにわたって私わたしは早起はやおきをしてきました。

   For six years, I was an early riser. That only changed recently.

2. そのパーティーではみなにいきわたるだけの食べ物があった。

   There was enough food at the party for everyone to have some.

spanning from one point to another

からにかけて expresses a span from one point to another, covering the whole range in between, whether time (years, months) or physical extent (waist to knees). It attaches to a noun followed by から, then にかけて; the first noun can be omitted when clear from context.

1. その役場やくばは１８９５年ねんから１８９７年ねんにかけて建たてられました。

   The town hall was built between 1895 and 1897.

2. 雨がよく降るのは、５月から８月にかけてです。

   The months with the most rain are May, June, July and August.

by way of; by means of an intermediary or medium

を通して attaches to a noun and means doing something through or via a person, thing, or medium, as in routing a complaint through someone. It also appears in the set phrase 目を通して, meaning to look over or read through a document. The examples use polite request forms, so it fits formal speech.

1. それは誰だれを通とおして苦情くじょうを申もうし出でればいいのですか。

   Through whom am I to make my complaint, then?

2. こちらの書類に目を通していただきたいのですが。

   I would like you to look over these documents.