wow, i had nearly this exact same question. i asked my premed advisor and he's been doing that for 30+ years so i figured he had some advice haha. i was thinking about delaying my April 5th date too and then he said that'd be foolish. he said if you don't get the score you want, then at least you find out in advance and then signup to retake it in like July or more likely August. although the first score may be bad, at least it is a SCORE and having one will allow you to still submit your AMCAS materials early like you want. AMCAS won't send out your primaries until they receive your MCAT score. then the schools will still receive your primary and send you their secondaries. fill those out and send those out asap too. then not too long after your 2nd MCAT testdate is coming up, you've had the summer to prepare properly this time, and then you submit your new July/August score to all the med schools you've already applied to. most med schools don't start looking at RD applicants till September anyway so you should be alright on time. you'd probably want to signup for a late July/early August testdate, though.
originally, I thought this was a stupid idea because I thought you should only take the MCAT once and when you're ready. the advisor said "yes you should only take it when you're ready but if you're already signed up it's foolish to cancel it now because if you don't get the score you want you can retake it and now you'll be more experienced with it, too." he backed up his claim that it's okay to retake the MCAT when he told me 80% of med applicants take the MCAT a 2nd time so it's completely normal. his exact words were "if you get a 36 the first time, don't retake it. if you get a 29, then retake it." anyways, i would strongly consider what i just mentioned to you. i initially thought what i thought because i gained most of my knowledge from these SDN forums. he immediately asked "where you are reading all this?? it's completely wrong." i told him adn he said "never take the advice on med admissions from doctors, medical students, or premeds. they do not remember the pain in the butt it is to go through the process." so yea, seriously consider what i told ya. i'd take his 30+ year experience as the main pre-professional advisor over some premeds and current med. students' advice!
the only flaws i see in his advice, though, are 1) if you're wanting to do ED you probably can't do this and 2) some schools i hear screen before sending secondaries so if you have a real ****ty first MCAT score maybe you wouldn't get some secondaries at all. i'm not sure how #2 works...maybe if they did screen you out but then you sent them a much improved July/August MCAT score then they would send you the secondary after?
I take issue with this advice for a very simple reason: Every score you take on the MCAT gets sent to medical schools. Let's see what this means for student A who took the MCAT while unprepared, and student B who didn't take the MCAT and prepped a little longer:
Student A- Your situation, planned, booked and paid for Apr 5, didn't study, and now 8 weeks before the exam is having doubts. Decides to go with the advisor's advice and takes the MCAT. Well let's say student A decides to take a practice test a week before, and scores a 24 on it. Its safe to say that student A will be getting a 24 +/-2 points on test day. Student A now has a bad score, with applications coming up soon (first week in June). He/She must now decide to either send in their app with a bad score (and risk immediate rejection), or wait until they take the test a second time and submit their application only then, with one bad score, and one -hopefully- good one and hope they take mercy and ignore your first bad score.
Student B- Same student, same situation, decides not to take the test, reschedules to another, reasonable, testing session ASAP, creates a study calendar, and gets a friend, family member, tutor, to help keep them on track. Student B schedules their exam for July, a little late since scores wouldn't come back until August. In early june student B sends in the application (schools don't look at it until the MCAT score comes in). With a good study plan in place the student takes the test and does relatively well (say an improvement of 5 points - a 30). Now the school has a hopefully good app, and a decent score to go by. No real flags, or black marks, and you're likely to at least get an interview.
The difference is that in situation B you're not putting extreme or undue stress on yourself by forcing yourself to dig yourself out of a hole. Truth is so many students need to be put in that situation of getting a bad score on the real test in order to be motivated enough to study (hence your advisor's remarks on repeat rates). If you feel like that's you, I suggest you go to your local Kaplan center, explain this situation, and they'll sit you in front of one of their computers for 5 hours to take a practice test. At the end of it just imagine yourself with that score on test day and internalize that feeling.
If that's not motivation enough, then you may need a tutor to push you along and make sure you're hitting your milestones.
Good luck.