My daugher is 17 months older than my twins. I nursed her throughout my high risk pregnancy (TTTS) with the full support of my perinatologist and OB.
Just because someone is a LC does not mean they know squat about tandem nursing OR nursing twins and a singleton. I had to bounce 2 LCs out of my hospital room because they gave me such incompetent advice it would have really screwed things up if I didn't know what I was doing. Nursing twins and a toddler is a very unusual situation--even at LLL meetings while people were supportive, they would also readily admit that they could only give general advice because they had no experience. So if you're going to do this, to be honest, I would not expect a lot of situationally specific help or advice--people are simply not going to know, even 'experts'.
I also think you may be overthinking things a bit. Yes, it MAY be hard for your toddler to adjust, but it may not be as hard as you think either. My kiddo was younger, but really after the babies had been here 2 weeks it was like they'd always been there. She was not freaked out about nursing babies. (She did totally wig out over the breast pump I rented when one of the boys was in PICU though--like, running screaming in terror from the room freak out. Which I can understand, really, pumps look and sound pretty freaky, especially the industrial ones, and I never pumped at all with her.)
I don't think sharing toys is an indication of how a kid is going to adjust to a sibling (either good or bad). Of course he's not sharing well, he's 2. You need to have realistic, developmentally appropriate expectations; he'll need some extra care and attention, but chances are things will be just fine. I found dealing with the hormone crash and sleep deprivation harder to deal with than helping my toddler adjust, to be honest.
So. What I would say is...relax. Dump any consultant that has a hardon for telling you to wean, you don't need that kind of agenda-driven ignorance. A good LC will work with you on specific issues (which I'm not sure you're going to know until you're there), so concentrate on finding someone who you connect with, and who can mind their tongue about weaning if you're not interested. I can't believe that any LC in their right mind is telling someone to wean their toddler BEFORE the babies come (barring some kind of medical issue), hello people, you think engorgement from ONE newborn is bad? You will never be so happy to see a toddler's eyes light up with milk-wanting glee as you are when your milk comes in and you have an expert someone to relieve the pressure so you can actually latch on your newborn. (at least, that is my experience).
That being said, I have heard that some toddler will cut down/self wean before the birth because the milk production slows down or there's an off taste, that really didn't seem to happen with DD (she would comfort nurse even though I don't think there was much there the last month or two). She literally did a jumping up and down happy dance (complete with cackles of joy) when she saw me in the hospital after the boys were born and she knew the milk was back.
Oh, also, we did the family bed with all three. We put the babies inbetween us, DD on the side (our bed was scooted up to the wall), later sidecar-ed a crib next to the bed for her when she wanted more space. No big deal.