Not sure if you are still looking for answers. I just thought I would post my thoughts here so that they may help someone.
I am not a legal expert. These are just my opinions.
- Can I use any bridge to cross?
Any bridge that has a CBSA (Canada Border Services Agency) on or along the way should be fine. It looks like this one will be on the way that you are talking about : CBSA, 5660 Falls Ave, Niagara Falls, ON L2E 6T1, Canada
- Can I just walk across instead of in a car? (I am planning to stay in Canada for few days. So I would like to rent a car on other side.)
Once we are inside the CBSA facility, we will go through the regular phases like being given a token, waiting in a seating area until our name or token number is called, walk to the conter where the an officer will process our documentation, fill out the forms, etc. (If you went to a US consulate in India for visa stamping in the past, it would probably be easier to think of this like that. Obviously, remove the feelings of tension from that analogy because in this case, it is Canada. How you go there does not matter - you can walk, take a cab, drive, etc. Once you are inside the building, it is a whole different world.) You might have to think about logistics like, is there a side walk that you can use to get there?
- How long does this take? (Especially because I am walking across)
I cannot comment about the time it takes to walk there. In my case, I drove in my car and parked it in the parling lot of the CBSA facility. Once I entered the CBSA building, it took me about an hour (give or take 15 minutes) to finish the entire process. After that, I went to a coffee shop on the other side of the border and after finishing my coffee, I turned around and returned to USA. I documented the experience here if you want to take a look. (Do I have to land to CoPR City Of Destination)