It all boils down on how the users does the "randomly" picking of words.
And the main problem are users not understanding things or doing bad choices.
We don't really need to discard first-last pages nor words on the page. That would be mostly useful IMHO for forbidding extremely bad practices, such as a user that chose as the password the first 4 words from the dictionary.
In fact, I would hypothesize that a user trying to come up with a random word would be more likely to choose a word in the middle of the page than at the top, since the top or bottom one looks "less random" (they should actually be chosen with the same probability). Similarly, the user will probably tend to select little-known words over common ones.
The page will probably come from "opening the book at random", which will add as another factor the wear down of the book and how likely it is to open on a particular page.
The beauty of using a dice is that it gives you a fair initial randomness: we assume that we have an unbiased generator of 1-6 random values as the seed of our password.
Not using a dice but letting the user randomly pick up pages and words we know that the entropy will suffer (which you try to counter by using a larger dictionary) but not really "how much".
There are a few tricks we could try to attempt getting the user to be more neutral, such as having them choose a priori the entry or page they will be choosing, before opening the dictionary and reading the page, but the initial issue persists.
I would recommend you to take a step back and formualte why you want not to use a dice. Perhaps it is problematic to roll it many times? You consider the diceware list unreliable? You actually don't have a dice available?
As an alternative of using a dice, I would recommend the following:
- Write the numbers 0-9 on ten cards
- Mix them and choose one of them. Save the card back. Repeat three times.
- Visit the page of the dictionary whose number is those extracted in the prior step (if the dictionary has less pages than that, discard that page number and start again to extract three values).
- Mix the cards and extract a couple of numbers. Pick the entry in the page with that number (if there are less than that, start again. If the page only has 50 entries, you may pick the first number from 0-4 instead of 0-9)
Here we replace the dice rolling with picking a card, which is also a simple model, as compared with determining how much randomness you lose by opening certain dictionary.