Shouldn't beneath work just as well as under?
Yes. Please report it if that comes up again.
IMHO there's no difference between "rest" and "have a rest" in this case and the second variant shall be accepted
How to change suffix of number? Sometimes I see húsz, and sometimes I see huszon
Húsz should be 20, 21-29 would begin with Húszon.
Same as for tíz when used on its own for "10", and tizen- when used as a prefix corresponding roughly to the "-teen" suffix in English, like in tizennégy ("fourteen").