Can I use "normal" shoes/trainers with clipless pedals?
Yes, you can use them with normal shoes, but as you predict, it isn't very comfortable, especially if your shoes have thin, flexible soles. Also, there's a risk of your foot slipping off, particularly in the wet.

There are various options to temporarily convert clip pedals into ordinary flat ones.


Fly pedals 
BBB BPD FeetRest pedal adaptors (SPD only)