I'm glad you brought these things to my attention, as it allowed me to add some things to my argument against Drill that previously were not included.
Some More Problems With Sand Rush Sets (I think there's just one general one in there as well)
Offensive Checks: For starters, I want to talk about offensive checks, as I didn't really cover them. I forgot completely about Rotom Wash. Assuming rock slide is used and not rapid spin, the matchup seems in favor of Rotom Wash. RWash switches in on a swords dance, and has to land a Hydro Pump through a rock slide, that, in order to OHKO, is a pretty substantial roll in favor of Rwash. While not ideal, the math seems to line up for Rwash winning, as hitting the pump on the Drill through the rock slide is about a 62% chance (+2 252 Atk Excadrill Rock Slide vs. 252 HP / 0 Def Rotom-Wash: 158-187 (51.9 - 61.5%) -- 96.1% chance to 2HKO after Leftovers recovery). And yes, Excadrill can't have sand rush and mold breaker at the same time, so levitate still works for it. AshGren, a very prominent mon currently, is a severe shut down to all Drill variants, as water shuriken is a confirmed kill on sand rush drill. Drill getting smacked by other weather is pretty self explanatory, as it can't really do much when a Pelipper comes in when it's time for a sand rush sweep. Zard-Y in particular shuts down a Drill even at +6, as it nullifies the speed boost and can't really be touched without rock slide, but can't be outsped without rapid spin (more on that later). Even things you wouldn't expect such as Mega Lop and Mega Medicham are good drill counters, as fake out taking away a turn of sand really helps to hamper what a Drill can do (sand is 5 turns, and you can't switch in Drill the turn sand goes up, so with a swords dance as well, it gets 3 turns to wreak havoc. Fake out takes away 1-2 of those turns usually, and then you outspeed). All in all, there are a host of offensive checks I didn't mention, and my bad for doing so.
Set Diversity Is Worse In Implementation: One of the greatest things about Excadrill is that it has 3 viable sets...in theory. From a team composition, it is very easy to guess what Excadrill came to the match at preview. Sand? It's pretty obviously sand sweeping Drill. HO? It's probably a suicide lead. Anything else? It's probably spdef with toxic. This probably seems pretty niche of a complaint, but when talking about a set diverse pokemon such as Clefable, it's extremely hard to tell what Clefable comes to any given match (not impossible, but significantly hard compared to most others due to it's extremely viable set diversity). One of Drill's main selling points (aside from lol sand rush) is that you can choose between a few cool sets. However, there is no surprise factor with which set it can be, so while it is good that it has set diversity, it's really only more choices in the teambuilder. This wasn't really a point brought up but I just thought of it.
Struggles to get to +2, and does not pull its weight: Nobody is going to sit and argue that a +2 Drill is sand rush is not one of the most horrifying things in the metagame. It is, and it has very little checks. With that being said, I think that in most cases, Drill doesn't get to +2 all the way. This is a purely hypothetical situation, but let's say that in an endgame, my opponent has a Tyranitar and an Excadrill. My HDB Cinderace goes for Hi Jump Kick to KO the Tyranitar, and looking to sweep, my opponent brings in Excadrill. Does he go for the swords dance there? I think he is forced to revenge kill the Cinderace with an earthquake and forgo the +2 for that turn. The Cinderace probably stays in and clicks hjk again, unless the opponent has enough turns to stall out the sand with sacks, and then Cinderace can come back in and hit (yes, HJK can miss, but I don't think factoring in hax or luck while judging the viability of a mon is a remotely good idea, and it opens up a whole different can of worms). There is a significant margin of pokemon Drill does set up on, but there are also a fair amount of mons that Drill struggles to set up on.
4 move slots hamper effectiveness: Slide quake, as you mentioned above, is a very smart way to deal with mons that are immune to ground and resist steel, such as the kanto brids without Articuno. With that being said, not being able to achieve a speed boost with rapid spin leaves it very susceptible to things revenge killing it once sand expires, which, to give a short list, is Victini, MMedi, Mlop, Cinderace, Garchomp, Weavile (knock is a roll, triple axel can miss), and with chip, aura sphere Mega Latias, + some others that I can't think of atm (I'd have to calc EP Kyurem). As you also said, not having rock slide leaves it susceptible to these 4x steel resist ground immune pokemon. Yes, there are other ways of dealing with this pokemon, and most of them are OHKOd by a +2 move under sand, but it seems like you're fishing for the opponent not having one of those speed checks if you're slide, and 4x steel resist if you're rapid spin.
Requires significant team support: This is pretty self explanatory, but Drill needs at least one sand stream mon to maximize its effectiveness in sand rush slots. Also, suicide lead is usually best on HO/balanced offense. Spdef Drill is the most lenient in terms of teamstyle (which is why I think it's the best), but if we're talking sand rush drill, it requires TTar or Hippo. Adding smooth rock to one of these to maximize sand turns is an option, but an option that will minimize the effectiveness of your setter, so it's a trade off that imo is not worth it and shouldn't be considered in Drill's viability.
There's a bit more I can go into, but it's probably beating a dead horse. Imo, when lined up next to Hydreigon, Fini, Lele, Tangrowth, and the horse, Excadrill is a cut weaker than those pokemon, and therefore it falls into B+ imo. Feel free to disagree, but I think that the evidence I have brought to the table is substantial enough to where this should be considered seriously. Enjoy!