Your PCs will likely not survive the Amber Temple fully intact. Even adding the two Winter Wolves is not likely to help, as they are only CR 3 creatures. That is, one Winter Wolf is a medium challenge for a party of 4-5 level 3 characters.
Compare that to the CR 12 monster in the first encounter, which alone would be a challenge for 4 level 12 player characters! Not only that, but the Arcanaloth is a 16th-level spellcaster, with finger of death and chain lightning both in it's repertoire, in addition to no fewer than 3 castings of fireball available to it. In 5 optimal rounds in this one encounter, you're looking at a lot of damage, certainly enough to kill at least one of your PC's.
Without doing the calculations (don't have my DMG at work...), two Winter Wolves are still only a hard (or possibly deadly) challenge for APL 3. You can't treat them as level 8 characters based on the hit dice, because level 8 player characters have classes which grant abilities at level 1, level 3, level 5, level 6, and so on, with level 5 being the most important as it's where the power level ramps up (fireball and Extra Attack come "online" at level 5). Winter Wolves don't have any of these advantages, and even with Pack Tactics and Cold Breath, it's unlikely to make a difference as the party is shelled by the highly deadly encounters within the Amber Temple.
The Amber Temple is one of two areas in the adventure with a recommended party level of 10, the other being Castle Ravenloft itself. See the Areas by Level table early on in the book (page 6). You can have a raven try to warn them away, similar to the encounter at the entrance to the windmill, but ultimately your players are the ones who get to choose.
That being said, if they are intent on entering the temple, let them. If they survive, so be it. If they start getting wrecked early on, you can hint to them that it might be wise to regroup and come back later when they're stronger, i.e., when they're more appropriately leveled and well-equipped. If they don't take your advice and push forward anyway, let them TPK.
In Barovia, the spell fails unless a specific condition is met
All that being said, in Barovia (emphasis mine):
Astral projection, teleport, plane shift, and similar spells cast for the purpose of leaving Barovia simply fail, as do effects that banish a creature to another plane of existence.
... so the banishment spell has no effect until:
Strahd is defeated, [when] the fog that surrounds Barovia fades away, allowing the inhabitants of the valley to leave if they wish.
It is unclear if that event removes the restriction to planar travel or only permits inhabitants to leave through that specific method.
It's unclear for shadow dragons in general
Shadow dragons, according to their Monster Manual entry are:
true dragons that were either born in the Shadowfell or transformed by years spent within its dismal confines
In the former case, their plane of origin would be the Shadowfell, in which case, they would not return, remaining on the Shadowfell. Along a similar vein, a dragon born in most other planes who then was transformed, would also be permanently banished to that different plane.
On the other hand, a dragon born in Barovia who was transformed from some other effect would return (after being banished to a harmless demiplane).
The shadow dragon in Curse of Strahd:
I could not find a reference to one, so it is possible your GM added it to the campaign. If that is the case, it is up to the GM where this specific shadow dragon originated from.
If it is, as you suspect:
Argynvost
... then the dragon's origin plane is still unclear, as no mention of it is found in the module except that:
he had come to the valley years earlier in the guise of a nobleman named Lord Argynvost.
Without any further details of his origin, it will be up to the GM to decide if the dragon is native to Barovia or a different plane.
Best Answer
We can't tell you
Balance doesn't come from the monsters you put on the table, it comes from how you play them and how your players play their characters.
If you play Strahd as written, a clever and diabolical opponent with centuries of experience of wiping the floor with adventurers then, unless your players are very, very good and highly experienced then they are dead PCs walking. See How can I play monsters and NPCs up to their potential?
For example, Strahd has these abilities that will make things really tough on the PCs:
Used judiciously, with that on the table, I could kill the party you describe with no trouble at all. Basically, I'd pick them off one by one.
Given the gothic horror nature of the adventure this would be a very nice end to the campaign: just when they feel themselves strong enough to take on the BBEG he teaches them a lesson in hubris and ruthlessness. Classic!
Alternatively, I could put a dozen minions on the table with them., employ them stupidly and give the party an easy win. but that would be boring.
You have the power
Strahd likes to play with his food so it is completely in character for him to devastate the party, gloat and walk away, just so he can do it again tomorrow.
Or weaken the party and then leave them to his minions in a number that you are confident the party can deal with even in their emanciated state.
And here is the little DM secret you need to know. Come closer, I'll whisper it.