3. In the second shuffle, double Steward is basically strictly better than single Steward if getting thin is more important than racing to $5. The collision case is basically the same outcome as opening Steward / Anything Else - you don't buy anything on the collision turn, you get something on the other turn. The non collision case is much better - you trashed 4 cards by turn 5. Nice!
I mostly agree with your post on why double Steward is a decent opening, but not this section.
Steward + other terminal is identical to double Steward in the collision case, and so it would take an extraordinary terminal to even consider opening with it over double Stewart (Peasant, Swindler, Black Market, Remake, and Jack of all Trades are some of the only terminal actions I'd consider opening with Steward).
But Steward + good non-terminal is often a better opening than double Steward. Cards like Tournament, Ironmonger, Magpie, and Wishing Well can give you a chance to get a $5 card and trash two cards on turns 3 and 4 - if they don't collide. If your openers do "collide", you'll usually be able to buy a $3 component on that turn. Those cards also reduce the likelihood of your Steward missing the shuffle.
Steward + Silver is sometimes better than Steward + Steward. If they don't collide, you have a good chance of reaching $5 in your hand with Silver. If they do collide, you can still pick up a $3 card while trashing. Which is better will depend on the rest of the kingdom. Strong $2 cards and plentiful action splitters favor double Steward. Strong $3 and $5 components and limited terminal space favor Steward + Silver.
Edit: Because Dingan is correct.