1.) You could switch to the intermediate plan. I am not 100% sure where your fitness is, but switching is an option.
2.) You could still do the run/walk progression, but it won’t be in the intermediate plan. It is only written in for the Base plan-you could make up a similar progression, though, if you wanted to get back into running shape.
3.) I am not sure if switching is a horrible idea as I am unsure what your training has looked like since Covid. If you are just now returning to training, I highly recommend staying on the base plan. I would add aerobic volume to some of the aerobic days but keep the rest weeks the same. For example (runs could be 45-1hr instead of 30-45) for the longer days, add an hour each week to what is planned. If you are recovering well, you can keep adding aerobic volume. Does that make sense? You could use this as a big base period and focus more on specialized training leading up to Aug 18th. August is still a ways away.
I hope that answers your question!