holy I think that's pretty much how it goes. GA kind of emphasizes what it's marking, answering the unasked questions we all want to know the answers to. (Though in your examples the questions are asked)
I think that's why the question words nani and dare always take ga instead of wa.
Counter = No Particle The first example Q&A should be;
Q: [color=red:6490f1c1ed]なんにん[/color:6490f1c1ed]、いますか。([color=red:6490f1c1ed]NAN NIN[/color:6490f1c1ed] IMASUKA) = [color=red:6490f1c1ed]How many people [/color:6490f1c1ed]are there?
A:[color=red:6490f1c1ed] ひとり[/color:6490f1c1ed]、います。([color=red:6490f1c1ed]HITORI[/color:6490f1c1ed], IMASU) = There is [color=red:6490f1c1ed]one[/color:6490f1c1ed].
No particle is needed after [color=red:6490f1c1ed]a counter/number[/color:6490f1c1ed].
As for your second Q&A, it is correct.
When the subject of the sentence is a question word (e.g. DARE, DOKO, NANI), you need to use GA instead of WA. The same particle (GA) is used when you answer. (See Course 1/Lesson 3 Grammar PART 4)