The last picture you posted is fine and this tub comes with a top plate that has a slight pitch to it.
There would be no water damage if silicone correctly.
What you don't want is a flat surround tub shower that will collect shower water.

A tub is installed over a sand or "mud" base to support it and the weight of the water. I really don;t think you would have settling, as your house is old enough to have settled fully. If you have settling issues once the initial settling has occurred, then you might have issues with the land the house was built on. Do you live in an earthquake zone?

If the home is still experiencing movement, structural supports may be undersized. Many older homes do not have beefy enough joists to be able to minimize deflection. Movement and tile does not mix. It cracks. If undersized, it will continue to move, which will damage everything that you put in that bath. That should be a priority to assess before you do any planning for a bath.

For your tub, the integral tub flange is a must. No undermount with ledge if you are doing a tub shower combo. You also need correct waterproofing for the walls installed down over that tile flange. Either pick the larger 66" tub with flange that occupies the whole alcove, or use the standard 60" alcove tub and fur out the wall at the rear, allowing for a large recessed niche in the tub area.

https://m.lowes.com/pd/KOHLER-Underscore-60-in-White-Acrylic-Rectangular-Right-Hand-Drain-Alcove-Bathtub/1000095624?cm_mmc=SCE_PLA_ONLY--

The second pic appears to be an undermount. Typically in this type of installation the deck is not provided by the tub manufacturer. As such, the deck must be fabricated, usually by those fabricating counter tops, out of natural stone or engineered quartz. Obtaining a slope into the tub may be impossible as they usually fabricate the deck as a single piece, no different than an undermount kitchen sink setup. Some fabricators may be capable of milling a slope into the top surface but they need to be good to get it right.

A tub such as that shown above by live_wire_oak with an integrated tile flange is most likely the best choice. These are available as shown so you may have a tiled face or with a matching integrated front panel. The former would look like the first posted pic.

Also, if you adamantly want an undermount tub and you can get the deck properly fabricated then you can create a suitable tiling flange using a product such as Schluter Kerdi-liner, they have preformed "L" shaped strips and corner pieces. These would need to applied to the Deck prior to installation using a high quality sealant. One flange of the "L" to the underside of the deck slab and the other "L" section extending upward like a typical tile flange. When the deck is installed the Kerdi-liner would then be tied into the wall waterproofing system. Tile the walls and you're good to go.

Thank you, I appreciate your information. Yes a tile flange sounds like the way to go. I am replacing an alcove tub shower with an updated alcove tub shower. The tub is oval inside which looks nice but takes up space, as I am tall and tired of having to bathe with bent knees sticking out of the bath. Yes the home foundations needed to be lifted, lots of cracks in sheet rock and the wood floor but not in the existing tile floor. This makes me feel OK about a tile flange. I will be replacing some sort of plastic solid surround with a tile surround.

tile flange