The only way I can make sense of the whole spice thing is by assuming that there are complexities to the spice trade that we're not even aware of. If spice shipments are just changing hands, why don't they offload it from one ship to another in the same space port? If it's highly illegal and thus dangerous to transport, who's actually enforcing the law there?
I figure it's less "illegal" and more "heavily taxed" (realistically, most drugs also have non-drug uses, like medicine or research; that's how people get the idea to
use them as drugs in the first place). Tatooine isn't completely lawless; the fact that Mos Espa has a mayor is evidence that there's
some kind of legal/administrative structure (with some sort of charter/constitution and a probably-anemic law enforcement arm). During the Empire, the planet probably had an Imperial governor and a tiny garrison (even if they were entirely under Jabba's slimy thumb), and we know the New Republic's been patrolling the sector (via Mr. Kim).
If they're taking inspiration from the old EU, Tatooine is between Hutt Space and a major trade lane (the Corellian Run), a nexus of multiple smaller lanes, and used to be part of Hutt Space at the start of the Clone Wars (Hutt Space has
greatly reduced since the formation of the Empire, but Tatooine itself clearly remained a gateway).
(spoilered 'cause maps are big.)
For that matter, we have no idea what "running spice" actually entails in this particular context; were the Pykes bringing it in from offworld and distributing it to third-parties? Were they
collecting it from third-parties and sending it offworld to be sold elsewhere? Were they brokering deals between unrelated third-parties (for a portion of the take)? Was this entirely a Pyke operation with no third-parties directly involved? Was this spice bound
for Hutt space (and being redirected by the Pykes before it could arrive), or was it coming
from Hutt space (having been stolen/smuggled out by third-parties).
For that matter, there could be a hidden spice stockpile or processing centre somewhere on the planet (say, at the far end of that train route), and the Pykes are collecting and then parcelling it out (possibly-raw spice gets delivered to the spaceport, transported by train to the hidden fortress until payments are made, then processed spice is transported back out by train to where the buyer can pick it up).
Also, to point out what too many reviewers don't seem to understand; characters in a work of fiction do not have foreknowledge. Just because we-the-audience know that a plan will fail after seeing the episode, the characters in-universe can't, and can only adapt desperately as situations change.
Fett had no way of knowing the Pykes were going to show up with a couple of Clone War-era war droids. They knew the bulk of the Pyke forces hadn't yet arrived through the port,
and they believed the treaty with the Klatooinians, Trandoshans, and Aqualish gangs would hold --they were watching for Pykes moving through those territories (who may have arrived elsewhere and been brought into Mos Espa specifically to avoid the obviously-watched starport), but weren't expecting the gangs
themselves to get involved. They thought they'd have time to receive a warning and marshal their forces.
Digging in at the (burned, but still structurally sound because the walls are made of concrete) Sanctuary made perfect sense at the time; they were expecting to face infantry, possibly with a few medium-scale assault vehicles. If Team Fett could hold out long enough, the siege would stop being profitable and both sides could discuss terms (which would give Boba an opportunity to target the leader).
Just before the shooting starts, Drash (watching the Aqualish workers' district) confirms that the Pykes haven't arrived, though she sensed something "off" (sidebar: is Drash Force-sensitive? In the old EU that would've been a tip-off).
Did Fennec act on her own accord? Or did Boba order her to do the wetworks?
Yes, that was the agreed-upon sequence of events; once they realized the attack was imminent, Fennec
said "we need to take out Command and Control," and Fett confirmed that the Syndicate is based in Mos Eisley (i.e., an
entirely different city from where they were), at which point Fennec leaps on her speeder bike and races off.
She stops on the way to rescue the kids, because Melinda May is the same in every universe, but gets back on the road and evidently spends the entire battle racing to Mos Eisley, where she
takes out Command & Control. The Pykes have clearly been monitoring the situation (and know how things have turned out), but as far as we can tell, Fennec probably thought the fight was still going on.
I found it odd that Boba couldn't rally or hire a single person in Mos Espa to stand up against the Pykes, other than the mods. Or rather, I found it odd that he didn't try. If the Mos Espa citizens are too soft or scared of the local crime bosses, he does have an entire hive of scum and villainy in his territory where he could hire a bunch of allies. I don't get what the obstacle was.
It was established several episodes ago that "people don't respect (i.e., fear)" Fett --what he's trying to do is a lot like the early Rebellion; people aren't going to support your cause until you've won enough victories to prove that you don't need them. The
smart money was with the Syndicate or the local gangs.
Remember that this is a planet that's been under (effective) Hutt control since before the Clone Wars (and Fortuna just kept the pre-established wheels turning). There are
generations of people there who've internalized the idea that they have no power to change things, and their only two choices are to keep their heads down or attach themselves to the Winning Team (and live like kings, at least until they get gunned down), and
that's the way things will always be. It's a foregone conclusion that Team Fett is going to lose (just like the Rebel Alliance) --until it
doesn't, and suddenly change is in the air.