Yes, I would say reading the entire passage first is to your benefit.
Most of the reading comprehension questions on the PCAT are not straightforward questions that you can dig straight out of the passage. They're often the "what's the general idea", "what can you infer" types of questions so it definitely helps to read the entire thing first, I find I did better by doing that. When I did the Cliff's practice tests I could get away with only reading parts of the passage that pertained to the questions because they were so straightforward but the PCAT is not so.
And I have the same problem as you for math right now lol... only thing I can say is that if you can't figure out how to get to the answer within your head in 5 seconds (like not actually doing the math, but the right steps to get to the answers) then move on to an easier question. There are probably at least half of the questions in the math section that are no brainer types. And just remember that every question of the 48 are worth the same mark so definitely tackle the easier ones first.