No, you cannot access either the circular terrace or the ruins below. The terrace is closed off. It was the king's prison at a time and still original jewels are there. Below that is the wall of the fort with three layers of what may look like ruins but were actually the security at the time. You cannot access either the terrace or the structures below.

Probably during rain season but maybe you don’t want to be there then?

I’m not sure there is an official time. We arrived a little before sunset (in December when days are shorter). Looks like they close the gates about a half hour before sunset and stop letting people inside. When we left around sunset, there were many people still there, appearing to be in no rush to leave. The guards simply opened the gate and let us out.

Off course yes, if river water is clean which is not mostly. Well, you can see the reflection of TAj in river only on moon light and that time entry to Mehtab Bagh is closed. So try other options too.

I don't think so. The river is very low, I can't see any real chance for that.

Nooo.... just take a cab and he will drop you. Agra is a small city, mac time to cross the city would be like 30 to 35 mins.

podés preguntar en español. la entrada al parque que esta frente al taj mahal es free pero no así al edificio...

That sounds great and quite impressive. Congrats Yes do avoid food and plastics

Safety: is always a precaution It would adviseable to avoid very lonely places inside the garden Try to walk with the crowd and at suitable timing, as very correctly said late evening should be avoided