My question is regarding the use of washrooms at workplaces :

1) Suppose if we haven't seen any najasat on the toilet seat then we should consider it as paak/ not najis and we can use it, but what if we know that a hindu has used it, even if there is no najasat, should we still make it paak? What about other public washrooms where we know that non Muslims are using it, but we cannot say exactly which one they used for eg. at a shopping mall

2) if we make it paak using the Muslim shower, do we have to put water 3 times? Or once is enough because we are using the Muslim shower the water is jaari/continuous?

1- If you are not sure that his body was wet, then you can assume that it is taher.
2- One time is enough.