If your long straightedge is bowing from the pressure of the router, you could support it in the middle by butting a two-foot-long scrap of particleboard (or almost anything, really) against the back of the straightedge and clamping it to the far side of the counter. That said, as long as the bow in the front edge is a smooth, even contour without abrupt jogs, it shouldn't cause any problem with the laminate. It might be better aesthetically if it were straight, but the laminate doesn't care if it's got a gentle curve in it. It's hard to imagine the counters are sagging on the sawhorses so much that they'll be permanently distorted by gluing laminate on them in that position. However, you'll need them fairly well supported for the glue up so you can lean some of your weight on the roller you use to press the glued surfaces together. For that reason alone it's worth supporting them well. You don't need a lot of sawhorses; I'd just put a couple of 2x4s on edge, on top of the sawhorses to provide even support along the whole length. You'll need to stabilize those 2x4s a bit, probably by nailing or screwing a couple of cross pieces between them, so they don't fall over flat at some inopportune moment. What size router do you have? There's nothing wrong with holding the router sideways except that you don't have practice doing it, and it could be difficult if you have a large, heavy router. The ones made for laminate trimming are small and light and can be held at odd angles with one hand. If you're stuck with a bigger, general-purpose router then it may be worth getting some help to set the pieces on edge as you described. You don't need a giant vise; just clamp it to some stable vertical object like a cabinet frame or the side of a sawhorse. When the clamp gets in the way, stop routing and repostition as necessary. Whichever way you go, if you've never done it before and you can't afford to goof, PRACTICE on some scrap!

Maeve- I am in the same position. While I realize many are steering you towards just buying new, I realize it's multiple steps to get a new countertop. Hence the reason I am avoiding it. My fix is for 2 years until I get new cabinets (lower) with a granite counter installed. There are trim options, even in ceramic, to create a lip (a little hackish) between the edge of the counter once removed. You could also hack it with a a bit of round (be sure to reinforce the finish when painted for water contact. Do you plan to RE-finish the counter top? I would assume you will need to relaminate or cement it. I am using a heavier finish. I am using this: Daich Coatings Inc.spreadstone ($80+) for a slate look, I am hacking their technique (emailed them to make sure it would work). Once the back lip/splash is removed it's going to leave some attractive particle and maybe some uneven areas. I have seamfill and putty to bridge the gap. Is this cheaper than a new laminate counter, yes. Is the time worth it, maybe not. I'm not sure about refinishing the counter with wallpaper (meant to mimic a finish) or laminate look sheets. Some of those are inexpensive.

The cabinets are semi custom and designed fine. I have to add a buildup as the style I chose has drawer and door facing that goes all the way to the top of the base to cover the rail. I like the look. I am installing a 3cm marble with a laminated edge on the island so I'm not worried about distance from upper cabinetry. However, the island is large and houses drawers down one side and doors around the other, plus a set of under counter refrigerator drawers... so the double edge will definitel obstruct the opening. My stone fabricator has informed me that they don't do the buildup and that it needs to be in place with the proper opening for the cutouts. They suggested building a frame with cross bars. I didn't think about the knotting of the wood so to me plywood makes more sense, but I've not had to consider this before.

"My question is: Is it possible to do a mitred edge on a countertop that already has a laminated edge?" No, not practically, especially with a stone with movement as you have. Those mitered edges have to be planned for before the first cut is made. That cutout appears to have 90* inside corners. If so, you're out of warranty. That, in combinztion with your stacked edge and mismatched waterfall mitered end, make me think your fabricator is completely clueless.