Seeming will not work to make a dire wolf appear as an orc
You can't change a target's body type, so you must choose a form that has the same basic arrangement of limbs.
This part of the spell means that while in wild shape the druid will not be able to take advantage of the illusion of being an orc since the dire wolf does not have the same basic arrangements of limbs (the wolf being a quadruped versus the orc being a biped).
You can make each creature seem 1 foot shorter or taller and appear thin, fat, or in between.
Even if they did have the same arrangement of limbs, transforming from a large direwolf to a medium orc would definitely be considerably more than 1 foot of height difference and arguably would exceed the extent appearing fatter or thinner as well.
If you had using seeming after wild shape to create an illusion that the druid was another type of creature with the same basic arrangement of limbs then it would work while the druid was still in that shape. There may be other shapes that could work (ape for example would be a tenuous but possible choice), but for the majority of wild shape -> orc transformations it just will not work.
Of course, in the end, it would be up to the DM to decide how strictly they want to interpret what is or is not the same "body type" or "arrangement of limbs". A sufficiently lenient interpretation may open up options. See this Q&A for further discussion on this point.
Seeming would probably be suppressed until the druid returned to its original shape (DM decision)
At that point it is not entirely clear RAW what would happen to the spell since the druid has effectively become disqualified to receive the benefits of the spell as it was cast.
However it seems a reasonable way to handle this is to say that since the druid has changed into a form incompatible with the way the spell was cast, the spell had no effect while it's in that shape. However as long as the spell is maintained, there is no reason that the spell can't regain effect once the druid becomes a valid shape again.
Jeremy Crawford offers the same suggestion in response to a somewhat similar question:
Q: Does Charm Person spell ends if Polymorph is cast on the charmed humanoid? Or does Polymorph suppress the spell called Charm Person until the human form returns?
A: There's no rule governing what happens when a valid spell target temporarily becomes an invalid target. A good rule of thumb is that the spell is suppressed while the target is invalid.
In the end, as this is a gap in the rules, the DM will have to decide if they take this suggestion or rule instead that the spell is dispelled.
The contentious part of this question is whether a plate in mid air can be considered 'ground'.
The Instant Fortress description says:
You can use an action to place this 1-inch metal cube on the ground and speak its command word.
I'd rule that a plate in mid-air could not be considered ground.
You've linked to another question on this site, to support your assertion that a plate could be considered 'ground'. However, in the accepted (and most popular) answer to that question, Vladislav Martin says:
'ground refers broadly to any solid continuous, contiguous surface on the planet.'
Which actually seems to disagree with your assertion completely.
While it's possible that a DM might rule in your favour (certainly for Rules As Fun reasons), probably most would rule, RAW,that a floating plate could not be understood as 'ground'.
That said, you were right, once in the air, there's no reason the fortress wouldn't be subject to gravity.
The Instant Fortress description also, says:
'magic prevents it from being tipped over'
But this doesn't logically imply that it couldn't fall - though it could be argued that it might fall in an oddly straight manner, landing straight and stuck like a dart in a dart board.
However, there is already a way to use this as a weapon, RAW
Each creature in the area where the fortress appears must make a DC 15 Dexterity saving throw, taking 10d10 bludgeoning damage on a failed save, or half as much damage on a successful one.
So, why not simply throw the unexpanded fortress down and then expand it once it lands, on the ground, next to the BBEG?
If it can't be a meteor, it's still a fairly impressive grenade.
At the very least, this is a great compromise option if the DM won't let you do what you really want to to.
Best Answer
This might work, but some things are up to the GM
You might be able to mount the spirit
The summon bestial spirit spell states:
From this we can conclude that the spirit is physically there and interactable (it is corporeal; it can be attacked as well). So now we just need to check whether it works as a mount. The rules on "Mounted Combat" state:
If you become Tiny (through Wild Shape most likely), then the spirit is one size larger so it qualifies there; however, it will be up to the GM whether the spirit has the appropriate anatomy to serve as a mount for whatever creature you turn into.
You can command the spirit while Wild Shaped if you can speak a language
The rules on controlling a mount state:
The spirit has an Intelligence of 4 but whether or not that counts as intelligent and whether or not the spirit has been trained to accept a rider will be up to the GM. Regardless, the summon bestial spirit spell states:
Thus it will at least obey the commands you issue. Notably, if you don't issue any commands it simply won't do anything, though it's a bit unclear how (if at all) that clause applies outside of combat or if the spirit is considered an independent mount.
Important to remember is that the spirit understands any languages you speak, so if you somehow turned into a Tiny beast that knows a language, you could communicate verbally while in beast form (if you somehow turned into a telepathic Tiny beast, the GM will have to decide whether you can command the spirit). In other words, if the beast you turn into does not know a language, then you cannot command the spirit.
What sort of commands you can give is up to the GM
Issuing commands is something the GM will have to decide. Can you issue a non-immediate command: "fly up in five minutes"? What about tasks that take more than six seconds: "bring me home"? Do you have to issue a command every six seconds? How does the spirit having only 4 Intelligence impact this? It's unclear how complex commands can be and what their requirements are, so it'll be up to the GM.
Alternatively, just get grappled
The Small spirit can grapple (as far as I'm aware) every official PC race. This would mean it moves at half-speed but it still allows for a rather sizeable amount of aerial movement. (60 feet per turn using the Dash action, and you have 1 hour so that's roughly thirty-six thousand feet covered). This avoids practically all questions about appropriate anatomy, controlled vs independent mount, languages spoken, and command complexity while also letting you keep a use of Wild Shape.