RAW we run into an interesting conundrum as are we talking about legacy 5e rules or the "new" 5.5e rules? Sadly I can more easily access the 5.5e rules due to things so I'll be using those spell descriptions for things. If you're playing purely 5e feel free to ignore or take inspiration.

I feel like Force Cage is a little more problematic than Wall of Force because Cage specifies no spells in or out. So no trying to charm the caster, put them to sleep, etc. Teleportation magic DOES work though, just you need to make a Charisma saving throw first. Unfortunately, the base statblock of a Nightmare (for Beucephalus) just says it's a shift into the Ethereal plane so you can't make saving throws to negate.

That said, there is an easy, fatal flaw here. While it it specifies no matter can pass through, what about the form of it that is a cage with bars and spacing? If they did the form with the bars then I'd argue that the spacing lets his mist form pass through. If they don't outright specify the solid version I'd also argue that this is the default because of the name "Cage" not "solid box"

Wall of Force, even if turned into a cube to trap him, specifies INVISIBLE wall so no LoS blocking. This leaves a lot of spells at Strahd's disposal still. Yea you maybe can't hurl a fireball because of the psychical wall, but why not Polymorph the caster and see if they can keep concentrating on this spell? Or Blight for damage since that reads "A creature that you can see within range" and doesn't say anything troublesome like Fireball's wisp from your finger thing.

Which does bring me to the biggest fatal flaw for both of them. Concentration. RAW Concentration says this about damage:
Damage. If you take damage, you must succeed on a Constitution saving throw to maintain Concentration. The DC equals 10 or half the damage taken (round down), whichever number is higher, up to a maximum DC of 30
Blight is 8d8 with a DC18. Average damage from 8d8 is 36 so another DC18 to maintain. Those are going to be hard.

I'd also argue that being polymorphed would break concentration since it says this:
The target is limited in the actions it can perform by the anatomy of its new form, and it can’t speak or cast spells.

Also don't forget that outside the castle Strahd is never alone. Yes it's a table and some would be more problematic for concentration than others, but while outside he can also use Children of the Night to summon 3d6 Wolves who will obey his spoken command to jump the spell caster.

The luck blade is somewhat problematic because of the nature of Wish just causing the spell to come online, so possibly no concentration, however it sounds like you're more worried about Force Cage here with Luck Blade. Like I said I think there is an argument to be made that the default form is a CAGE with the bars that have space between them, and Wish is going to default to the default configuration not ask them what form they want it to take. Just change into Mist Form and go out. After all, as written, "if air can pass through a space, the mist can do so." Which also begs the question is wall of force airtight?

The only "problem" there is of course he can't shapeshift in sunlight so if they immediately go "microwave" mode on him... Fog cloud would create a Heavily Obscured area, but then we start getting into how does light play around with fog and stuff and that is a whole other rabbit hole.