But my questions remain unaswered.
Is there a certain threshold where Core devs will add native UASF support? when is enough support "enough"?
No. Core does not just implement something because some arbitrary and easily faked metric (node count is highly unreliable due to the easiness of making thousands of "nodes") shows that "enough" people support a proposal. Many Core devs think that UASF is unsafe anyways, so unless the proposal significantly changes, it probably won't be implemented into Core. There is currently no indication or talk about implementing UASF into Core.
Is uacomment same as running the UASF client?
No. The UASF client will enforce segwit signalling after August 1st. That behavior is not present in Core.