With regards to your first question, you probably should make that an option or at least query the user on which piece(s) of information they've forgotten. If they forgot their password but remember their security question then it doesn't make sense to require a new security question answer. If the user forgot both then they'll need to reset both.
You could prompt them for both pieces of info, but not require answers to both. So if a user enters a new password but not a new security question answer you will only store the new password and leave the old security question answer record alone. The user could also provide both pieces of info and in that case you update the user records for both.
Security questions don't really work well with the reset model. You can't expect them to choose a different answer for the same question so that means changing questions as well. Depending on your site, there may already be a limited number of questions, which the average user will narrow down further depending on which ones they can answer or want to use. So forcing a change with every password reset can cause new answers to be even less memorable.
Likewise, the 'memorable word' is essentially just another password. So if you reset it every time a password needs to be reset you're giving the user two new pieces of information to memorize. It goes from 'memorable' to 'a random word I was forced to come up with'.
For your second question, the only security merits of resetting the answer or word is if an attacker has obtained those pieces of info alongside a valid password. If a user realizes this theft has happened they may want to change passwords and security answers. There may be situations where this is true, but I wouldn't expect that the majority of user password resets will be associated with this type of compromise.