You may have figured this out by now from the contracter, but countertop people should do plywood underlayer. It has to be to guarantee the right fit to the quartz - too many things can go wrong if someone else does it. They screw the plywood into the cabinet. They use a epoxy-like cement between the quartz and plywood - they will float the slab on a large amount of gooey glue, then while the glue starts to cure, they adjust the height with wood screws from below to level precisely. Then the glue sets within 10-20 minutes or so, then it's done.

Generally there is sufficient means to secure the counter without having access to one area so there should be no problems with what you want to do. When you install the oven, however, pretend that the counter is in place. One does not want to build in a problem that can only be solved by removing the counter. E.g. conduit or pipe too short, reachable from above, but can't pull oven out later. Also, protect the oven door handles or remove the door to avoid damage. And, finally, tape some cardboard over the control panel to help avoid scratches or dings during the counter install.

I don't know what you intend to do regarding backsplash height (that's the distance b/w the finished counter and the bottom of the upper cabinet and light rail), but here's what I usually tell people when they're considering the height for their backsplash. The standard is 18"...regardless of the reasons why, it's still the standard.. How this affects your kitchen.... Refrigerator and other tall cabinet heights...Because wall cabinets are meant to be mounted at a standard height, cabinet manufacturers take this into account when designing tall cabinets. Tall cabinets are designed to be the same height as the installed wall cabinets so the tops all line up. When you change the height a wall cabinet is installed at, it affects cabinet top alignments. With the refrigerator you can usually mount the upper cabinet a little lower OR order a shorter upper cabinet, but be sure you don't make the alcove any shorter than 72" tall b/c newer refrigerators are 70" to 72" tall (and seem to get taller each year!) With other tall cabinets like oven cabinets and pantry cabinets, they're a standard height and designed so they're the same height as the wall cabinets when those wall cabinets are installed 18" above a 36" high counter. If you have custom cabinets, this may not be an issue b/c your cabinetmaker can adjust the cabinet heights. But, if you are using stock or even some semi-custom cabinets, you cannot change the height. You can often get taller cabinets for use w/36" or 42" tall wall cabinets, but not shorter for 30" mounted lower. But, even those that are taller are also adjusted based on standard wall cabinet heights + an 18" backsplash height. Small appliance height...Small appliance manufacturers often design their products to meet this 18" standard...for example, my KA stand mixer (bowl lift) is around 17" and many coffeemakers and blenders are just short of 18". So, you need to be sure you have room under the cabinets + light rail to fit those appliances. Small appliances... Keep in mind that if you're using an appliance on your counter, you must be sure you have room under the base cabinets to move that appliance around easily. Don't, for example, tell yourself you're only going to use it in front of your upper cabinets so you don't need to worry about its fit. In reality, you will be moving things around on your counter while you work and most likely your appliance will be moved under the cabinets at some point. You don't want to tear your light rail off or damage it (or the cabinets). Also remember that "standard" upper cabinets are 12" deep + 1" for the door...so they stick out 13" over your counter...leaving you only 12" or so of workspace in front of the upper cabinets...not much room to work in by itself! (If you have deeper upper cabinets...say 15", there's even less counter frontage in front of your upper cabinets...3" less, so 9".) Vertical workspace... Another consideration is vertical work...

I would strongly recommend AGAINST buying ANY CounterBalance product... they will completely hose the homeowner / DYI consumer. Just compare the price for the brackets between http://counterbalanceshop.com and http://www.chemical-concepts.com/counterbalance.php The same product will cost you $114 at the first site and $81 at the second. Really??? THEY ARE BOTH THE SAME VENDOR. I tried to place an order based upon the info on the Chemical site (didn't even know about the counterbalance site) and they wouldn't honor the price posted. Don't do business with this vendor who isn't interested in doing business with you. Vendor's words: "I could care less about" selling to consumers. STAY AWAY !!! Find a local machine shop who is interested in doing business at a fair price/profit. The product is a VERY simple job for any machine shop to produce. Chemical concepts doesn't even make the product -- they sell glue. For the person who wanted to know where they could get metal support bars, here is an option: http://metalworking.mscdirect.com/CGI/MWSRCH?N=32003649 I don't have any experience with them, so I don't know if their prices/policies are good or bad. I just found them from doing a bit of searching.