Newly found dog may be pregnant

About a month ago (27 days to be exact) we found a dog that was seemingly dumped in our rural area near a lake. She was very sweet, looked to be around 3 years old, and had enlarged nipples from what seemed to be a previous pregnancy. We decided to take her in to look for a microchip and check if she was currently pregnant. Long story short, there was no ultrasound technician at our vet in Nashville and they were booked for quite some time going further (they only do ultrasounds one day a week). We decided to just get her vaccines taken care of and then schedule a spay appointment at another vet closer to the area we found her in. The county provides vouchers for stray dogs to be brought in and spayed.

However, over the last week while we were waiting for our voucher to come in the mail and then schedule the procedure, I've noticed a weight gain and a firm belly. Now theres really no doubt in my mind that she is pregnant but I'm calling tomorrow (Monday) to see if I can find an ultrasound technician ASAP to confirm.

The point of this post is for advice, really. I've taken care of many dogs in my life, but never puppies. And never a pregnant dog.

Is an ultrasound able to identify the age of the puppies? Will they have an accurate timeline of when she will be close to labor?

Does the mama need a special diet going forward? Her exercise has been regular. We hike/walk with her daily, usually twice a day if not more.

The Internet has expensive whelping boxes on Amazon, but if anyone has any suggestions on how to create a safe/CLEAN whelping area, let me know. Do we just use a tarp, blankets, and puppy pads or is a whelping pad the best idea?

After the puppies are born - how much care and attention do they need from us? Is the mama allowed to still go on walks? Can we leave the puppies alone for certain periods of time? I read somewhere that a heating pad can help keep them warm - if there are any product suggestions for this as well.

I think at this point I'm just overwhelmed. We've grown quite attached to this dog (we named her Lady) and it was always in the back of our mind that she may have been dumped because she was pregnant, or the fear of pregnancy, but she was so skinny when we found her that I thought the odds were low. But shit, she could have mated a day prior to us finding her. Who knows.

ANY ADVICE will be helpful. I was writing to gather some knowledge from people who have gone through this experience before.