A:
You can set up a command that creates an array of responses from which Nightbot will pick from at random. Here is a model you can follow:
!addcom -cd=5 hi $(eval responses=["Response 1","Response 2","Response 3"];responses[Math.floor(Math.random()*responses.length)])
Copying and pasting this into your chat will create a command hi that will output one of the responses in the array when a user in the chat says “hi”. You can create another command that does the same thing by simply changing the command name hi to something else like Nightbot. You can edit the array as you see fit. Just make sure that each response is bound by straight double quotes "" or straight single quotes '' and that the responses are separated by commas. Also make sure the entire command response, starting from $(eval to the closing parenthesis ), is less than 500 characters. If you require more character space than that, you can use ehsankia’s quote system CustomAPI to store responses on a list from which Nightbot can pull a response at random.
B:
As far as I know, you cannot make a timer activate at random intervals of time.
What you can do, though, is set the quantity of chat lines that must pass before the timer can activate (which, by the way, also cannot be set to fire at random quantities). The logic here is that a chat usually follows an irregular pattern. Sometimes it can be busy with many chatters sending messages in a short amount of time. Sometimes it can be slow with just a couple messages being sent every once in a while. That means, in theory, setting a timer to activate based on the quantity of chat lines passed will make Nightbot feel more like a viewer rather than depending on the timer to activate at a set interval of time.
Another thing you can do is make a timer that outputs a random message every time it activates. Create another command similar to what I have set up for you (the command that uses an array of responses). Create a new timer and put the command name in the field labeled Alias. Then put $(query) in the field labeled Message.