It is a bit of a risk, but not really. If you carry Dugtrio as your answer to Scarf Sleep, I will be able to run free with Raikou after I Pursuit trap your Dugtrio with Swellow (for example). For my Raikou team in particular, that's not a bad trade. However switching Dugtrio in on Venusaur is pretty risky in general...it is a grass-type.
Although use of CB Dugtrio may have risen recently with the departure of Honchkrow, I personally still think it is a bad idea. Life Orb + Substitute + 3 attacks is the far superior build IMO, and any offensive team is going to feel the pain if that thing manages to set up on Scarf Venu's Sleep Powder. I don't know of a single offensive Pokemon that can handle Dugtrio behind a Sub carrying Earthquake + Rock Slide / Stone Edge + Sucker Punch without taking at least a heavy beating, and with the scarfer already dispatched. Perhaps a Swellow with prediction using U-turn and Quick Attack, but even that is a mindgame at best.
And generally super-effective rarely applies as significant for a Pokemon as frail as Duggie. It really needs to avoid any kind of heavy hit at all costs.
And I don't believe I ever used a Lum Berry Mesprit...
Well maybe not Lum Berry, but I remember you mentioning the Mesprit + Dugtrio combo as a way of dispatching lead Froslass. The same setup can be used to capitalize on Scarf sleepers with a simple item change.
This isn't to say Scarf Sleep isn't risky, but is there a better/alternative way to beat Froslass, Rain, and Dual Screen leads? I don't believe so.
A difficult question to answer, given that I don't even believe Scarf sleep is a
good way of beating those. But in general I see that as the wrong way of approaching the problem. A well-built team should be well prepared to take on any of these strategies at least adequately as they come. Being so paranoid and even
reliant on preventing the setup altogether is very dangerous IMO, especially with a strategy that has less than 50% chance of working, as I have already shown. I prefer to just keep smacking them hard so that whatever they do they can't come back and do it again. As an example, a simple lead Spiritomb + offensive anti-rain Pokemon (Toxicroak, Kabutops, Qwilfish etc) deals with those three mentioned problems by themselves almost completely, and I have had next to no problem with any of them using this and similar simple combos.
I personally think that those kind of setups are more dangerous when unexpected in the lategame anyway, where you may not have your best counters left. No anti-lead is stopping those.
Attacking a 1 HP Froslass, again, comes down to prediction. I'd personally switch to Venusaur again, because either way Froslass is going to either Destiny Bond or get another layer of Spikes if it wakes up. If it doesn't I can just Sludge Bomb it to death, otherwise I can sleep it again. However it is 100% prediction otherwise. If Froslass doesn't get Spikes down, it doesn't do its job anyway. Froslass should always get Spikes up against a non-centralized lead. Venusaur seems to be the only exception to this, even if it isn't foolproof.
So after already getting a lucky break with the first sleep duration, you think the best course of action is to try and do the same again? That is called pushing your luck, and at that point we are talking about success rates as low as 10-15%, much lower than the chances of getting a kill with an OHKO move, or hitting a Pokemon with +6 Evasion.
Please give me an example of an offensive team that can take 2-3 Air Slash / Fire Blast / Hidden Power Grasses from Moltres? Nothing can! There are honestly like 3 "semi-offensive" Pokemon in the top 50 who can switch in and beat Moltres safely, Azumarill, Life Orb Milotic, and HP Rock Houndoom.
Well there's DD Altaria, who can take a resisted hit and set up. Or Ninetales, who can set up after Fire Blast or kill with HP Rock. Or pivot Regirock (the best one, though I almost always prefer Registeel myself). Anyway, you're right, there aren't many
guaranteed switch-ins to Moltres on offensive teams, but then again they rarely need to have one, as they are all about playing to resistances. Coming in on a predicted resisted attack than threatening immediately usually suffices in difficult situations. It isn't the same as a stall team which relies on guaranteed counters to function properly at all times.
Also, I agree that Scarf Venusaur is best off with +Spe as a lead. But that also highlights the fact that it doesn't hold a candle to the great Roserade as far as scarf goes, lead or not. In addition to not having some great 'sure thing' support options against obvious wall switch-ins, Venusaur also lacks the power to really hurt and is forced to run +Spe on top to outrun every non-scarfer.
