2. Its grammatically correct to say "我在走" but it sounds better when its a "pair" so colloquially it uses a 词, which is a pair of characters which may just say the same thing. you could also say 我在走着 which adds no extra meaning but pads it to be a pair
3. 了 is meant for something in the past, past perfect tense is not the same as past tense, past perfect tense is about ambiguity not time.
4. 们 has the 人 radical, so its mean for humans. Otherwise you're to use counting particles instead, plus 很多 or 几 or some other particle to emphasise it.