I would characterize 我还没去过北京 as past perfect tense but in the negative: if "I have been to Beijing" is past perfect in the affirmative, then "I haven't been to

I would characterize 我还没去过北京 as past perfect tense but in the negative: if "I have been to Beijing" is past perfect in the affirmative, then "I haven't been to Beijing" is past perfect in the negative. Maybe a more straightforward example I should've gone with is "我去过北京" for "I've been to Beijing". Regarding 我在走路, yes, 路 acts as a filler for prosody purposes. If the prosody is fine, then you don't need it, e.g. 他走的很快 (swapping 我 for 他 here because it sounds kind of weird to say such a sentence about yourself) takes no noun just like the fragment 他在走, but mustn't use the filler noun 路 (他走路的很快 is incorrect). "我卖了这些车以后" I think is more like "After I sold these cars", where "sold" is definitely past tense here. "他要吃了吗？" expresses "Is he going to eat?", where 吃 hasn't taken place in the past.