when do you know plants are dormant ?

I'd forget about returning any plant anywhere, a reliable nursery will advise over the phone or email, can be slow as they are busy people. If there are problems they won't want the plants back anyway.

Why? a good nursery will always supply what you have ordered if it is in stock or advise you if out of stock.
Would you pay for genetic testing it's also not quite as simple as most people think, many species, let alone varieties, have never been tested so what do you compare your results to.
I don't know who you get your plants from, but I would stick to good nurseries that you have used before rather than going for the cheapest you can find.
As for your apple tree wait until the leaves drop then prune as previously advised in this thread.

 

We have already explained why pruning that particular type of apple tree may not be such a good idea and why a bigger pot would be better. Ignore that advice if you choose but don't be surprised if it goes wrong.

I sometimes buy plants online because this particular area is not full of nurseries and garden centres offering a wide variety of interesting plants. They can be very bog standard and not that cheap either so I buy some plants online.

So far I have had excellent results either with the quality of the plants receibed or with their response if things go wrong. EG i ordered a yellow flowered Magnolia Butterflies from a company on the far north east of France and the relay place here mislaid it so it spent a week in a box in the dark and was very sorry for itself when finally found. Instant reimbursement. They are a reputable nursery and rely on their reputation.

As @NigelJ says, going for cheapest is not always best so, for future purchases, check their online reviews before ordering.

 

I think there may just be a confusion as to what you have actually purchased. The problem can be how these are advertised, they normally use very specific, best case pictures, of columnar trees that are covered in fruit from bottom to top. This isn't the reality unless you develop the tree specifically in that way.
Patio apples are on very dwarfing rootstock but can still easily reach 2m. The trouble is they are designed to fruit quickly and heavily but to the detriment of lifespan and stability (they are advertised as always needing support).

If you want it shorter, take the top off but it will need yearly attention if you go that route to keep it in shape.

 

You can return items bought online but you would need to check their T&C's and the "guarantee period".
Obviously far too late for the Apple tree as you have had it for too long so your only options are to give it to a friend/neighbour, prune as advised or lop the top off. Your choice.
Just for the future........if you want to grow something which you have no experience of, try checking out the relevant specialist nurseries or societies for as much info/advice as possible before buying.

 

You can; however if you were a nursery would you want to get back a sick/dead plant, possibly carrying a disease or pest and almost certainly unsaleable. Surely you would simply apologise for the error ask the customer to dispose of the complaint and send them a correct replacement or refund their money if they preferred.

 

I think usually there is a time limit on returns and even refunds.
I doubt you would get either if the complaint was, its grown too big.

 

@NigelJ and @pete - agree with both comments - the OP's tree is presumably healthy but if it is doing what expected, no valid reason to return it other than a bad choice on the part of the purchaser. There are also time limits on returns and refunds as I have already pointed out hence my suggestion of the options open to @hi2u_uk .
As @Thevictorian mentions, the photos shown when some trees/plants are advertised are not always "realistic" and customers can fall for those tactics.
At the risk of repeating myself, it pays to obtain as much info/advice before purchasing anything you are unfamiiar with - plants included.
End of as far as I am concerned