Your d3 in the probiotic mix can work with Tums for now, but it is easier to keep some calcium with vitamin d3 tablets for later use. The egg looked a bit like the egg within an egg phenomena. Odd eggshells can be a onetime thing, but could be a sign of past infectious bronchitis virus exposure, stress, or infection. Egg shell problems are common in chickens. The link above for common egg quality problems is a good one to see how many defective shells and what may cause them. My flock once had infectious bronchitis, and so for 9 years, I have seen odd shells and later, salpingitis, and other reproductive problems. I would try the calcium and the amoxicillin to see if it helps. Sometimes I think that some sellers will pass on their hens with problems. Hopefully, your seller did not do that.