I need a to scale floor plan of the spaces you are working with . All meaurements clearly marked . Post that here in a comment. Do not start another post. Moving toilets is always a big job in this case you will need to find out where your sewer line is and how to hook the toilet into it .Why make your life harder by adding this bigger vanity ? You need to talk to a plumber to see what can be done . At least you are not moving too far away from the original drinage and sewer. BTW We have never had 2 sinks in a bathroom and we have been married for 60 yrs so IMO this would not be a driving force .I think a much bigger issue will be some plan as to aging in place for your parents since IMO wheelchair accessability is huge deal for bathrooms for sure . Of course you also need a shower in that bathroom space . Maybe you need to get some pro help .

Considering your scope of work, moving the toilet doesn't appear to be too much of an addition. I'd try to find the plumber who did the work. That could save you lots of concrete repair.

Anything is possible given enough money. You'll have to decide if it's worth it to you. Can you tell us why you want two sinks in a basement bathroom? Is there going to be a bedroom downstairs that will have two people in it all the time? Maybe an in-law suite? If it is an in-law suite that will only be occupied part of the year and the couple hasn't specifically requested two sinks I would just move on from the idea to reduce the complexity of the project.

Not a big deal to move the drain.

Just break the concrete.

Then cut away any embedded mesh, if the builder even bothered with it in the first place.

Any plumber who makes a big deal about that isn't a pro. Or takes you for a fool.

Any time you're dealing with concrete slabs, moving plumbing is not an easy fix. They would have to drill out the concrete in both the existing location and the new location. If you're set on it, get an estimate for the change order and then determine if it's worth it to you to pay the extra money. But if you're going to do it, now is the time before they finish the framing.

Breaking up the concrete, moving the plumbing, and re-pouring the slab in that area is not a big deal in a project like you're contemplating. Much better than compromising your design. Cutting it with a concrete saw will make for a neater looking job, but you can also just drill holes at intervals to define the perimeter and break it with a sledge, or rent an electric jackhammer-- they're fun to use. Presumably you'll be putting some form of flooring over it, so a ragged seam won't matter much. Basement floor slabs generally aren't very thick. Dealing with the floor can certainly be a DIY job to save money, but since the plumbing is hard to get to you probably want to hire a pro for that.

Cutting it with a concrete saw will make for a neater looking job,

Likely, concrete dust throughout the house! Neater yes, but for what purpose? It will be under whatever finished flooring you decide on.

Really think about the two sinks.

They are often not used in master ensuites. Really a waste of money IMO.