"And I’m not sure this is even a real problem." The membrane will provide waterproofing. The caulk and grout? For the moist part, they are there to help the surfaces shed water and to help maintain sanitary surfaces. You've gotten some good answers. Changes in plane are typically caulked. Caulk is flexible and can move when adjacent surfaces move independently of one another. Grout is rigid and can crack. Cracks in grout can be from movement...typically in corners, from differential movement...or because too much water was used in the grout mix or during clean up. Cracks in caulk are usually shrinkage cracks. Not enough caulk was injected in the crack for the caulk to properly bond to the adjacent surfaces...Or too much caulk was injected in a deep crevice, and as the too-deep mass of caulk cured and shrank, the shrinkage forces were strong enough to break the bond between the caulk and the edge of the tile, or they caused the caulk to crack right in the middle of the bead itself. For deep gaps that need to be caulked, a foam rope (backer rod) is inserted into the gap to reduce the depth of the gap. That allows the caulk to be injected in the gap at the proper width-to-depth ratio. Caulk texture? There both color- and texture-matched caulks these days. So if you have sanded grout, you can use a sanded caulk to help maintain joint texture between grout and caulk. Unsanded grout, use regular caulk. Repairs? In general, it's not wise to shmear a thin veneer of caulk over cracked grout. It's better to cut the grout away as best as you can and then properly fill the gap with color- and texture-matching caulk. With a Schluter shower, great care needs to be taken in cutting grout out of the joints. Go too deep and you can violate the integrity of the membrane. Your wall corners will have membrane behind them, with the membrane bridging from one wall to the other. In a non-steam shower, there is a chance that your wall-to-ceiling joints do not. Depends on the habits of the builder. The sooner the repairs are made the better. Grout hardness, ease of color-matching, etc. Good luck with your repairs.

You are correct in that using a drum sander is not at all DIY friendly. But it's the only way to correctly sand a floor down to smooth and ready to finish. You will take years trying to do the same thing with an orbital. The rental rate for the sander is going to be more than hiring the right pro with the right equipment.

The "Questions" are meant to be a field guide, to assist consumers in understanding the important qualifications. Our research indicates that some consumers (understandably) lack knowledge and experience when it comes to hiring qualified tile contractors. Some of the questions would likely be answered with basic research. The best Pros in the industry respect that a potential client has educated themselves. The worst will run for the hills as an expectation is being set that they might not be able to meet. Better yet, the more the consumer is educated and expects quality work, the greater the pressure for a contractor to seek the training and credentialing needed to perform the work to industry standards. It has to start with established expectations. And to your point, M Miller, while at this moment all trades are busy and consumers should plan ahead and be prepared to pay for quality that lasts, that is another piece of advice. And cpartist, #1 and #7 are extremely important in our experience. Speaking to #1, too many people without dedicated tile experience and training, and #7, experience with basic tile formats but not necessarily the tile chosen, like glass, natural stone, large format, etc. are resulting in problems. That being said, the vast majority of tile installations are meeting standards. We hear only about the "problems" on these forums...they are real and if consumers and the industry work together to expect qualified labor and results, we can continue to progress for the benefit of all. Truly appreciate the comments.

A utility knife can make metal marks. I'd suggest a used credit card. Something stiff enough to remove grout.