Greetings everyone,
I'm thankful to already have been learning a lot looking at other forum members posts but despite being very comfortable with hand layups and standard vacuum bagging procedures I'm still pretty intimidated by VARTM. I find myself continuing to learn more and more little details online that emphasize there is no room for error with one small detail or mistake likely to make or break my VARTM hull extensions project I am planned for my sailboat. I would like to talk through what I am thinking of so as to get any and all constructive criticism on my following thoughts and am grateful for your time and knowledge:
(I'm giving as much info as possible in the hopes of those more knowledgeable than I catching even the smallest of critical errors I might be making...)

First off, I'm planning on starting off with a number of small VARTM small trials and then plan to VARTM flat 4'x8' sheets of divinycell with various thicknesses of glass or basalt fibers on both sides simultaneously that can be used for assorted renovations on my boat as simple sheet goods. Then I intend to ''graduate'' relatively rapidly to my primary current goal of doing two transom extensions for my 40 foot catamaran (already removed 19 inches since they were marine ply given the rest of the hull was corecell, and then plan to be adding back those 19 inches plus about 30 inches that extending the existing hull lines, so I figure going with VARTM inside and out of roughly what will roughly be a “U” shape but with semi-hard chines, so overall be about 49” long with a max vertical draw of resin of 60” which occurs over actual vertical height of 48”).

General details and questions:
1) Planning to use a 6 CFM vacuum pump which is capable of drawing down to 20 microns with a quart resin trap. Using Aeromarine 300/21 epoxy which is 600 cps at 77 deg F (I have extensive personal experience with this specific epoxy so comfortable with working times relative to temperature and like how it definitely thins out a decent amount more just before finally kicking...). I intend to infuse into two layers of a Quad-Axial Basalt Fabric (weight 680 grams/sq. meter (19.9 oz./sq. yard), width 127 cm. (50"), Fiber orientation 0 degree, 90 degree +45 degree -45 degree. Made with 13 micron roving) with a 1” Divinycell H-80 foamcore.

Prior personal bagging experience has been with the venturi vacuum generators but so upgrading to this two stage rotary vane vacuum pump will be a new piece of kit for me but since unlike vinyl ester I don't have to worry about the epoxy boiling off at higher vacuums it seems pretty plug and play.

Infusions will likely be happening at somewhere between 77 deg F and 87 degrees F and I plan to test against having any leaks by sealing vacuum line off for one full hour prior to being confident enough to mix the epoxy if haven't lost vacuum. My understanding is with experience I could reduce the amount of time that I test to confirm vacuum is held prior to infusion but I should do an hour minimum starting out just to be on the safe side?
Sound reasonable so far?

2) What is the best way to create perforations and/or flow channels in the divinycell and what are the specifics of spacing and dimensions? IF this is the best route to take? This is where I am really out of my element and cant tell if 1/16” drill bit and circular saw with an 1/8” kerf going down 1/8” is reasonable, or deeper thinner cuts with a razor blade, or ____? Also just to confirm that this is a good path to take and worth the effort instead of using plain H-80 panels that aren't prepped in this way? IF instead using divinycell as is with flow media simply placed above the fiber reinforcements and later removed, and then simply prepped the surface of the divinycell with epoxy thickened with microspheres (with some 1/8 inch groves being previously cut to increase shear strength once thickened epoxy was hand applied...) and then applied dry fiber reinforcements and VARTM materials in the window of time that the microsphere thickened epoxy was mostly cured but still capable of primary bonding?

If preparing divinycell with perforations and flow channels is a flow media still needed or beneficial? (I realize they make some pretty swell divinycell sold specifically all ready for infusions but I already have a bunch of standard divinycell H-80...). If using using flow media should I go green resinflow 75, Enkafusion or___?

3) I am planning to use an MTI resin infusion control valve, and the MTI suction hose in addition to a spiral hose for vacuum. (Thinking being the MTI suction hose won't pull as fast due to the membrane?Also gives option to pull off excess resin if necessary?) With the two above vacuum hoses running along the top of the transom extension and resin input spiral wrap on the bottom exterior and interior of transom extension being basically a big U for first phase and then stairs and boxing everything in being added in an additional easy steps. Best way to make sure don't have a resin poor layup in terms of when to close the resin infusion line? (assuming leaving the membrane vacuum line on the entire time and closing the non-membrane vacuum line ).

Would really appreciate advice on optimal layering order and material choices/use of the following materials I have available: stretchalong 200 center fold, perforated PTFE fiberglass, generic breather fabric, polyester peel ply, perforated release film (and still to be acquired flow media if needed in addition to grooves in divinycell?). If using enkafusion flow media I am especially concerned about protecting from pinhole leaks but I would rather error on the side of using a bit more resin and materials to start than having the entire part be bad. Same goes for how long I should leave the resin line open after closing the spiral wrap vacuum line (and assuming leaving the MTI membrane suction line on the entire infusion?) to ensure I don't end up resin-starved anywhere?

Thanks in advance for any jewels of wisdom you are willing to share because I definitely still feel like I have more questions than answers before tip toeing into VARTM and sorry for an exceptionally wordy first post. 
Dave