Can I still apply N400 at my GC if I’m still married to the same American citizen but we don’t live together anymore?

Although eligible (assuming you meet all eligibility requirements), you are not obligated to become a citizen. But I do recommend you do so. If your spouse wishes to take that step and you're not ready, you don't have to. I do wonder about you and your spouse living separately. There may be a question as to marriage fraud, so you'd probably benefit from speaking with an attorney on this matter.

If you are no longer in a bona fide marital relationship even though you may still be legally married, you have to be a permanent resident for 5 years prior to qualifying for naturalization.

It depends on the circumstances of your case, but at a minimum you become eligible to apply for naturalization after you have been a lawful permanent resident for at least 5 years. Since you are no longer living with your USC spouse, you should not apply for naturalization on the basis of being married to a U.S. citizen for at least the last 3 years.

Generally, yes, as long as you have been a permanent resident for 4 years and 9 months and are otherwise qualified.