As far as the opening goes, you can put in a collapsing ladder, such as this one. I've installed one before and they're not too bad, just a little framing work and some large nails (I used 16p nails and you'll need a friend or two to help, the ladder and frame is somewhat weighty). This gives you a ladder and an easy way to get up to the attic and it collapses down to just a panel which can be painted and framed to make it look nice against the ceiling.
The attic in my house just has some plywood on the truss floor which serves as a great storage floor. Just have to be sure not to block the air flow in the attic and realize that it may get very hot in the summer and cold in the winter. I made the mistake of storing some candles in boxes one summer ..... wound up with candle blobs when I discovered them again.
Since you are in zone 4, R-38 is a reasonable insulation level. That doesn't tell us how much an issue a lack of vapor barrier is. It is less of an issue in dry climates. What a barrier does for you is prevent moisture from condensing inside the insulation, significantly reducing it's insulating value. Ever notice how hypothermia victims have invariably gotten themselves wet? Unfortunately, properly installing one can be very difficult (read: expensive). As it happens, plywood acts somewhat as a barrier, depending on it's construction. Thus, you may need to just focus on gaps and openings.
Another possible make-do vapor barrier is a good heavy coating of latex paint on the ceiling. You may already have this! Truth be told, the best way to insulate would be to remove the plywood and blow in the needed insulation. This gets sealed around all the joists and other ins and outs nicely and makes effective use of what little insulation is already there. However, it may not be worth the effort.
You can still get a good insulation job by layering it on top as you suggest. The draw back is the dead space under the plywood effectively becomes part of your conditioned space, greatly diminishing the value of what's there. Even a plaster ceiling alone is worth something. And you've increased the top story volume you're heating 5-6%. Another issue with this approach is the plywood becomes your vapor barrier. What are the ramifications of moisture condensing on the under side? There's poor air circulation, so it will not dry easily. It could be the cause of a mold colony gaining a foothold, which can have serious health consequences. Or it may not be an issue if you're in a dry climate.
Whichever way you go, try to ensure what ever passes as a vapor barrier is a complete as possible. Ensure any recessed light fixtures are not insulated over, and that no insulation contacts the cans. It may seem more efficient to cover them, but you greatly increase the chance of them overheating and starting a fire. Insulation is little good if your house burns down! A truly efficient ceiling has no recessed light fixtures for this very reason. Note that there are supposedly fixtures rated to be covered, but I don't think I've ever seen one.
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That would depend on how good the foundation is. Normally, the foundation is adequately overbuilt that it's not a concern. The weak link in this sort of conversion is USUALLY the "attic" floor above the garage wich is typically marginal even for light storage, and far from occupancy loads, over a very long span. Bringing that up to an adequate structure is a significant project if it was not built as such from the start.