Working with fondant

........... a few recipes that call for an instant pudding mix. Unfortunately, I can't find it in stores. Can I use a homemade version in the cake mix?

  Most cakes do well without adding pudding. 

I get the impression you are not in the US so I have no idea what a homemade version would consist of but I bet it would be just fine.

2. I levelled the cake after resting for half an hour or so and it was a big mess....Should I freeze the cake or put it in the fridge to firm up.

          Chilling the cake (either fzr or frig just until it is well chilled) would be helpful.  Most likely the cake was not cool enough and that's why it gave you so much trouble.

3. The weather is usually hot and humid and I found the icing a bit of a problem. Also the cake layers seemed a bit tilted.

           Keep your icing in the frig or on a bed of ice to keep it from melting. 

Use a cake saw to level your cake layers.  If even one layer is not perfectly level the whole cake will be off :(

If you don't have a saw and don't want to invest in one then use a knife that is as long as the cake is wide.  Leaving the cake in the pan, pass the knife over the pan and it will take off any excess above the level of the cake.

4. .....cake back in the fridge after applying Fondant covered in cling film.......the fondant was sweating.
.....how to properly prepare my cake for fondant, storage after applying fondant .........

          Put the finished cake in a cake box then put that into a plastic bag.  When taking from frig, leave in bag at least 30-60 minutes.  Sweating is natural.  The cake will be fine if it is NOT touched until all sweating has evaporated.

You actually probably won't save much time.  While the cake is baking & cooling you can easily whip up a batch of b'cream.  The canned icing can be part of what you use but alone it is very soft.  The more you stir it up the softer it gets :(