Yes, it certainly is!

Yes, for sure. Simply because the "s" sound in the beginning has a sibilant sound and no vibrations in the vocal cords and the "th" sound in the end also has no vibration at all. So we can conclude that only the middle of the word is voiced! I hope this explanation will do! Cheers!

Yes, it is one syllable. It has one vowel sound /e/ so it couldn't possibly be anything other than a single-syllable word. A word with just one vowel sound can only be said as one syllable. 'Strengths' is also one syllable, but 'strengthen' is two syllables, 'strengthened' is also two, but 'strengthening' is three. Try saying those three words: strength, strengthen and strengthening, and you'll hear that 'strengthen' has an extra syllable (+ en) and that 'strengthening' has an extra two (+en +ing). One bonus piece of language trivia. Did you know that, at eight letters long, 'strengths' is the longest one-syllable word in the English language?

Yes, although I will say that the piling up of consonants does produce some sort of sonic jumble that isn't quite a pure clean single syllable. I can say "One! Two! Three! Four!" in a fast, rhythmic cadence, but "Strength! Strength! Strength! Strength!" is a bit of a tongue-twister. I can't imagine a crowd at a rally chanting it. It's not unusual for people to pronounce it more like "strenth" if they are speaking quickly and casually, and if you say it clearly and carefully, I think there's a hint of a schwa sound after the "g." As a matter of fact, I see that ahdictionary.org considers it to be one syllable but shows no less than three pronunciations: strĕngkth, strĕngth, strĕnth. The first is what I called the "clear, careful" pronunciation, and the third is the "casual" pronunciation!