Nice testing and discussion. I don't play D2r, but only mentioning D2LoD.
DClone: once it says DClone walks the earth, it will appear (replace) a Quest monster... like Tearhaunch Act 1 (the Tree to get the scroll) ... and the FASTEST one is act 5, NOT Shenk, but go North and the pack there, that boss will be DClone... so WP tele north once or twice and Dclone will be there. There is also Act 1 ... Boneash... you have a wp to get there and 5 teleports North where Boneash is to spawn.... Dclone will be there. NOTE; you can not repeat Dclone spawning in the same game, also, if you are in Act 4 and completing the Seals and you see "Diablo walks the Earth" ... you are fighting Dclone, so if you have NOT done the quest and kill Dclone, you have to re-run Act 4 to kill Diablo.
Oh, some times with people popping in and out of your game and you are in level 4 to spawn Andarial... he won't spawn, it is a glitch that hardly ever happens, however I am talking about LoD.
Basically all bosses will be spawned once you enter their Domain... just like Ubers, but they are randomly spawned at different distances.
Lillith is a prime Uber to find her in a closed off room is normal.
Uber Tristram is simple... Meph North East, then go south to spawn Durial and you find Diablo basically inside Tristram.