Regarding your questions -
1) Like the others have mentioned the filter should come right off, if not ..... even after applying a little force, then it should be sent to a repair center for removal. You don't want a jammed filter on the front of a lens, you always need access to the front element for periodic cleaning. You can definitely mount a shade onto a Hasselblad filter if that's what yours is, it also has to be undamaged for teh shade to bayonet properly.
2) If you are used to the 4x5 upside down and reversed imaging, it should be no issue dealing with a simple reversed image on a Hasselblad. I use a PME3 prism, but, I still use my waistlevel the most. I agree completely with QG about the EV settings, much easier and faster to use. I have not used the PME45 so can't comment on it objectively except I can easily say it is quite pricey. The older, but great prisms are quite a value. They work beautifully but you would have to decide if a metered version is important to you. Personally, when I use a prism I want it to be metered. I use my PME3 with a Accumatte D in my bodies. I also agree with QG about the split image, I always use it, no matter what finder I have installed. I actually love using the magnifying chimney finder for images that are stationary and when I'm working off a tripod. I was real careful when I was looking for my prism and I found the PME3 to be the best value available, bang for the buck.
3) In my opinion, there is no question the Accumatte D with split image is going to help with focusing more than anything else. I have used an Accumatte with the micro prism but I did not find it helped nearly as much as the split image. The caveat though, is you must keep your eye well in alignment, centered, over teh split image or half of the split image will black out. A little practice and it becomes second nature.
4) The Hasselblad bubble level accessory is essential in my book if you are shooting in the field. I love that little gizmo. It is compact enough to easily put away and it never gets in the way when shooting and it is easily viewable when working above the camera. It's a fantastic accessory for the camera. There is also a flash shoe mount but I never use that but perhaps a wedding or event shooter may. There is a external battery holder that can be used for different body and accessory versions. There is a side mount sports viewfinder that fits too. There may be some other gizmos that I am forgetting which fit also. You need the Compendium book by Nordin to get a full rundown on the Hasselblad System, it will help you immensely along with Wildi's book.
5) Okay, your question leaves many options, you have to decide what your needs will be. No one can really give you a specific what is best answer to the question you posed because of the great variety components available. I can tell you what I use with my Arca B1 head. For the Blad (500 C/M's and 553ELX) I use Wimberley plates. What I do is use two of them in combination with two Hasselblad tripod couplers, basically the Hasselblad factory version of a quick release. I use two Hasselblad #45129 tripod quick couplers. I like that particular coupler the best of all of the versions they have made because the release lever and safety lock lever are on the same side of the coupler. I have one of these couplers mounted to a Wimberley P10 plate. It facilitates two things, the coupler allows quick removal of the camera body from the coupler leaving the camera body without some stupid chunk of metal attached to it. The coupler itself has a permanently mounted P10 plate mounted to the bottom of it which quickly releases from the Arca B1 head. I can swap bodies back and forth to the ballhead in a heartbeat and I can remove all mounting plates from the bodies in a heartbeat too. Best of both worlds and it works perfectly. Now, I also have a longer Wimberley plate (P50 I believe- kind of like a simple long macro rail) which is mounted to another Hasselblad coupler (45129), I sometimes use this in situations where I need quick forward and backward adjustment of camera position without having to move the tripod, all I do is loosen the Arca B1 clamping knob and slide the Wimberley plate back and forth for a quick camera adjustment. This allows quick mount and dismount of bodies as well. Works like a charm. I also use a Wimberley P10 plate on a Zeiss 500mm telephoto which I then clamp into the B1 which balances the rig nicely. I love Wimberley plates, I have never had an issue with them and they mount perfectly to the Hasselblad couplers and lens foot. The safety stop screws in the Wimberley plates are a big plus too. This entire setup that I have described will leave your Hasselblad in a stock configuration, nothing permanently mounted and hanging off the bottom of the body. The camera simply mounts and dismounts in a split second onto any Arca style quick release bracket ballhead no matter the brand without leaving a plate permanently attached to the blad body.
6) Yeah, you probably forgot a lot. Welcome to the club .... we all do.