Tiling over the bathtub lip vs up to the lip
I have good news for you - it does not matter!
Whether you tile over or under the lip (it's over and should be obvious why), you will absolutely be destroying the bottom of the water proofing of that shower/tub system - either way.   There is no way you are getting a tub out and a new one back in like they are lego blocks.   Good luck, never seen it - ever.
So you would rather install your tiles wrong and chance having water issues on the walls and have to take down an entire wall or a few walls, so you might be able to replace tub?   Hmmmm.
OK so if you are hazy on this tub.   If it is of low quality or you don't think it will last, you should spend money on new tub before doing anything to your walls.   You can get a good cast iron tub used for 300-400 plus another 300 to reface it.   Minus a refacing every 10-15 years it should last forever.   Tubs and shower pans are not items that can be swapped.   They are the bottom edge of a waterproofing system and once you try to swap them you take a huge chance of water issues - and thats given you have the room to get old one out and new one in without having to rip things down.