You're missing one thing:
You've used two actions in number 3.
You sheathe your rapier and draw a dagger/knife/dart from some sort of storage. Sheathing the rapier can be done for "free" (first bullet of "Interacting With Objects Around You," PHB p.190). But then drawing the dagger/dart would require the Use an Object action, per "Other Activity on Your Turn," same page. Attacking with the thrown weapon also requires an action, but you're out.
Your proposed scheme had you making six attacks in three rounds. I think you can use the same thinking and still respect the action economy by making one slight alteration. You run into the same draw/sheathe economy problem in a hypothetical fourth round, but the fight's likely over by then. So here's your original scheme, with a small modification:
Load and equip a hand crossbow before combat, along with a rapier in the other hand.
Attack with the rapier, follow up with crossbow as bonus action, without disadvantage in melee range, both due to crossbow expert feat and then move away from the enemy, avoiding opportunity attacks due to the mobile feat. Now sheathe the rapier, for "free".
Draw knife/dart for "free", throw a throwing knife, dart etc., and follow up with crossbow as bonus action, as the crossbow expert feat lets me ignore the loading quality of the crossbow as long as I have a hand free, which I would after throwing the dart/knife.
Fire crossbow as bonus action, still ignoring the loading as I still have a free hand, and then drawing my rapier and attacking, as drawing attacking counts as same action, and then move away, ignoring opportunity attack from mobile feat.
It is ok for level 5
Let's compare all aspects of the spells
Damage type
Poison is the worst, while magical bludgeoning is the best.
Stampede wins by a large margin.
Range
Same initially, Stampede can reach much further in the end.
Stampede wins.
Area and damage
These are hard to separate.
Stampede does more damage per round, if the target falls prone, but it can only damage an enemy multiple times if it runs in the direction the charging animals do, and about the same speed, meaning away from you. If they are running away, you already won.
Cloudkill moves much slower, a stationary enemy will remain in the area for several rounds.
Cloudkill wins by a very large margin against most enemies.
Duration
Most encounters do not reach the fifth round, and on average are about 3 rounds. So both will last the whole combat.
In my experience combat encounters are mostly further apart in time than 10 minutes, closer to 60. If they are more frequent, it usually happens because the party is running somewhere. These two together mean that area spells most of the time last a single encounter, regardless of duration.
About as good as Cloudkill
Unless I know that the Second Goblin Army is coming, I will prepare Cloudkill. It has a horrible damage type, but I will simply use something else against undead and elementals.
But at least it has a quite good chance to damage my enemies more than once.
Best Answer
Sure.
You can describe your Mobile halfling's movement however you want; something like parkour or freerunning is certainly an option. Just remember you don't get any other benefits beyond what is defined by the feat; the DM could reasonably call for Dexterity (Acrobatics) checks if you want to move in ways not allowed by Mobile.
Your breakdown of the possible move-attack-move combination for a rogue with Mobile is correct. The DM might well put you in circumstances where you can't always take advantage of this, but that's part of the fun.