Can 曾 be used alone instead of 曾经？
There are many verbs and adverbs in Chinese that hold the exact meaning without the second character. Chinese words have a more complete sound to them when they are in pairs of two characters, but it's not necessary to use the verb or adverb in its complete form.

我也曾(经)涉足于多个领域，可惜学业不精。
他已(经)20岁了，已(经)到及冠之年，是个大人了。
好久没有这样安恬熟睡(觉)了。

Having the two different choices though gives the speaker more control over making the sentence sound more fluid, generally by pairing single character words with the single character form, and using the full verb with two character form.