> So if I query for the A record of foo.example.com will it be answered by ns2?
Yes, if I understand your example correctly.
> If so can I assume that the .com tld server has entries for both example.com and foo.example.com?
No. The .com zone only contains NS records for example.com. The NS records for foo.example.com exist in the example.com zone (and, for consistency, in the foo.example.com zone), but not in the parent .com zone, which knows nothing about any sub-zones.
Yes, if I understand your example correctly.
> If so can I assume that the .com tld server has entries for both example.com and foo.example.com?
No. The .com zone only contains NS records for example.com. The NS records for foo.example.com exist in the example.com zone (and, for consistency, in the foo.example.com zone), but not in the parent .com zone, which knows nothing about any sub-zones.