It may be possible to cope out the top edge of the cabinet base to let the edges of the sink overhang the top edge of the cabinet. I had to do this myself and it worked just fine. See what the widest part of the sink it's self measures (not including the flange) and see what the inside width of the cabinet base it.

Sorry, but sink manufacturers write specifications to allow fat plumber hands access to provided clips to be able to tighten them. If you strap your undermounted sink (Hercules Universal Sink Harness or Sink Strap), the clips are eliminated along with the reason for accessing them. The sink is tightened at the bottom which anyone can access no matter how fat their hands. This allows a much smaller sink base cabinet to be utilized than specified. I rarely advise disregarding manufacturer's specifications/hardware, but clips are a poor method for attaching undermounted sinks. We wouldn't have methacrylate seams in solid surface or thermoforming if fabricators hadn't disregarded manufacturer's specifications. The profit motive births innovation. I'm installing sinks that other fabricators say "Can't be done." There is money to be made doing that. On a drop in sink as I pictured, I couldn't easily access the clips in the middle of the sides, only the front and rear. Yes, Kohler wanted the side clips installed. Boo hoo; that sink doesn't leak or budge.

I don't think you need to move anything. Flex lines for the water supplies and a different PVC fitting or two to accommodate the changed location of the drain should take care of it. There's no reason that something's not going to work because you've moved connections by a few inches.

Our hall bath always had 28" for the toilet (5.5x13' typical bathroom layout with vanity/toilet/tub), and the toilet was a bit off-center so it was not an equal 14" on each side.* The vanity always felt a little close on one side but not abnormally so, and I'm certainly not thin anymore but it has never really been uncomfortable in any way, even with countertop overhang included. I never realized it wasn't up to code until I measured in preparation for the remodel. I debated with myself quite a bit whether to get a built in vanity that was 24/12 that would be 39"-ish and allow a nice roomy 36" for the toilet (the first proposal from the cabinet seller) or a 24/16 vanity that would be 42"+filler and be pretty much what we had in place already (in terms of length) except we might gain an inch or so of room. Then I found out it was going to cost $760 to move the sink plumbing over slightly for the smaller vanity. And I thought about how narrow a 12" drawer can be (they're not actually 12" inside of course). And the cost for a 24/12 vanity was going to be about the same as for a 24/16 vanity. And I thought it's never bothered us since 1976 having a 44.5" vanity there and this vanity will actually end up being at least an inch less. And it's pretty open on the tub side of the toilet since we have a shower curtain and not glass so it doesn't feel claustrophobic to me. So I ordered the 42"+filler vanity and am very happy with it now that it's installed. I think I gained up to an inch on the narrow side by the vanity. (I weigh more than 250lbs so am not a small woman.) After the remodel I measured and now we have 30" between the vanity and the toilet rather than 28, but the toilet is still off-center so only 13.5" from the center of the toilet to the vanity. Certainly not ideal, but it doesn't bother us. And we've passed village inspections, etc. The next issue I had to deal with was what to do about toilet paper access. No room to mount a holder to or even recess a holder in the side of the vanity. Everyone (contractor and cabinet seller) kept telling me to get a TP stand, but I don't like most TP stands. So in the end I mounted a TP holder vertically on the wall between the tub and toilet (it's actually the same TP setup we have in our weird afterthought basement bathroom, so it's a comfortable reach for me). I know on some level the "right" thing to do was to get the smaller vanity, but there were so many things I liked about the option I chose instead. So either way I probably would have been happy. Photo that shows how close the toilet is to the vanity and my TP holder placement (maybe a cautionary tale for some people?): We're not concerned about resale as we plan to live here at least 10 more years and we remodel mostly with ourselves in mind since who knows what the future owners will want to redo a decade from now. *It was like this when my parents bought the house in 1976. The house was built by the original owner in 1959 and we only just learned this year during bathroom demo that originally the layout was toilet/sink/tub, so I'm actually glad the owner switched things around prior to 1976.