Ferrothorn is slower than Samurott. If Ferrothorn uses a non-attacking move(like Stealth Rock), Samurott will force him to SR the next turn as well, it's great if Ferro forces someone out, SR on the switch, and gets Encored.
Ah, true enough. Though more and more Ferrothorn seem to be running spikes it seems. Encore does let you get three free ice beams/blizzards, or if nothing else, a free switch.
Overall Encore helps prevent you from being an easy target against Ferrothorn, but if it just decides to power whip you from the get go, it can be a real pain. Though switching in on something like Gyro Ball and Encoring it is great, especially since Ferrothorn doesn't usually switch AWAY from a water type. Thanks for pointing that out, forgot about that. What with Whimsicott being an Encore Prankster, I forgot that you could also just Encore things if you out speed them anyway.
Samurai Audience Samurott probably wants wish support if it's running life orb, and it can appreciate wishes no matter what really, since it has decent bulk to survive neutral non-STAB attacks. I'd probably go with leftovers most of the time since life orb can set you open to revenge kills.
Still, against some staples of Sandstorm teams, like Gliscor, Skarmory, Hippowdon and Tyranitar, it stands quite bravely and can handle them. Against rain teams, it gets a very nice boost for it's STAB, but typically a rain team runs water types who resist it anyway, but it's not too bad. Specs Tornadus can pick you off if you've lost some HP due to Life Orb. Scizor doesn't want to switch into Samurott either, since Bullet Punch will be weak, and it will often be outsped.
On HP Vs. Speed IVs, it really depends if you think it's necessary to outspeed Breloom. While you could probably KO Breloom with Ice Beam / Blizzard, you run the risk of letting it spore if it wins a speed tie, which not only means you dying, but it can also mean free set up for your opponent.
Against the Lati twins, so long as you're facing them unboosted, you can probably survive things like stray Dragon Pulses, but should either carry Thunderbolt, you've got to be careful. Latias has an easier time dealing with Samurrot too since it has more sp. def and cares less for un-STABed ice beams. Though Blizzard still makes a serious dent. It's probably just a better idea overall to switch out when they're around, though you could technically switch into things like surf/hidden power fire which they are likely to carry. Dragon Pulse probably won't OHKO, but specs meteor from either probably will.
In terms of Hydro Pump Vs. Surf Vs. Scald, it depends on what your team needs. Spreading burn is always a nice thing, but missing out on the power to KO things hurts, especially since some opponents may see Samurott as set up bait, and if you don't predict the set up and encore it immediately, it can mean eating a sweep. If you are good at Encore prediction, Hydro Pump is probably your best option due to free turns meaning you have some leeway to use Hydro Pump. Encore however does cause switches, and burning things as they come in is always nice. Surf is a pretty decent middle ground and should always be considered if you need some reliable damage.