YouTube is full of tutorials on how to apply crystals in many different ways. For future, I suggest you watch some of them and try out which way works the best for you. However, keep in mind it takes time to master crystal application, no matter which method you choose. It took me about 4-6months to perfect my application to the point where clients rarely loose their crystals and they still give out the bling.
I have personally found that top coat alone does not do the job for me, as it is too weak to hold the jems in place, especially on hands.
I have a soak off gel that I use for crystals. File off gel could be used as well, but it can be more trickier to remove those crystals afterwards, especially if we are talking about genuine Swarovskis. Another alternative is gel polish base, but it has to be of a thick consistency.
So I do the following: perform the service (manicure, extension etc.) apply my last colour coat (usually gel polish), on top of it, where I want crystals to be, I place bead of soak off gel/gel polish base, the I apply crystals, cure it in the lamp twice as long as recommended for specific product, then I cover the nail with top coat, avoiding top coat application over crystals themselves, and fill in the spaces with top coat between crystals using small brush. Turn the hand over (so nails would be upside down - this way top coat floats into all the nooks and crannies between crystals) and flash cure it, then again place the hand in lamp as you normally would and cure it for full cycle. Woila - you are done! I do one finger at the time.
However, if I have to do full coverage Swarovski nail, some steps have to be adjusted. If need be - you can message me directly, I will tell you then how to do it.
About clients - if you are all booked today, but you have to take in this one to do the fix, maybe call some clients that you have later today and politely ask them to come in 30mins later, as you are running late on "someones service". Then there you will have 1/2h to do the fixes for this client. Clients usually are understanding if you inform them before hand.
I hope this helps.
Best of luck!