I've done WW casting with Alumilite clear, Alumilite white and PR. I get the best results from alumilite because I find it adheres to the wood better than PR but I have made some very successful WW casts with PR.
I do think pressure is necessary to avoid small air pockets in the blanks and help get the resin into small cracks and voids. I have been successful in getting the resin to penetrate into cracks as small as 0.1 mm without air pockets.
How you position the wood in the mold and how you pour the resin can help with good penetration into the voids as well. Put the wood in the mold in such a way to minimize the spots where air can get trapped. When you pour the resin, start pouring in an open area of the mold and let the resin flow through the mold from the bottom to the top. This way, the resin will displace the air from the bottom and displace it through the open areas left above. If you were to pour the resin through the tops of the voids, you'd risk trapping air bubble within the voids.
I don't think the short open time of alumilite is a significant issue as long as you plan ahead and have everything ready to go. That being said, I usually only cast one mold at a time and if you were trying to pour multiple molds you might have some more time constraints.
I make sure to have the wood in my molds ready to go (I use dots of CA to keep the wood from floating) and the pressure pot open and ready to put the lid back on. Measure side A of alumilite, put in your colorant(s) and mix well to distribute the colorant. Make sure to add enough colorant to make the final cast the right color as side B will dilute the color a good bit. Next, I add side B directly to side A and wix well for at least one minute. I know we're working against the clock here, but mixing the resin well is extremenly important, make sure to mix in all of the corners for the container, too. When everything is mixed, pour ino your molds as I described above. Once the resin is poured, I will usually tap the mold on a solid surface for 30 seconds or so to try ot release any trapped air bubbles. This helps especially well if you have small cracks in the casting wood. Then place the mold in the pressure pot, secure the lid and pressurize. I ususally can get the pot pressurized within 4 minutes of adding Side B so I've still got a 1 minute buffer at least. The key is really planning and having everything ready to go.
Make sure to leave the mold in the pot long enough to allow the resin to FULLY harden, alumilite clear will set quickly but I have found it is still fairly pliable for a couple of hours after setting. I have taken molds out of the pot and mold after just one hour and some pressurized air pockets that were left expanded since the resin was still pliable. I leave mine at least overnight to avoid this.
So there's my thoughs/process. Hope it helps.
