Just wondering; If a player on an entry level contact is assigned to the AHL team before the start of season, does he still get his signing bonus… Or is the signing bonus only activated if he makes the NHL team’s opening roster?

Yes that player gets the signing bonus on July 1st regardless if they play on the AHL or NHL.

To add to this question, What happens if that contract slides? Do they get a bonus signing bonus?

The signing bonus on ELC's is limited to $92500 and is paid regardless. If the contract slides, it is still paid that year, but there would be no bonus in the 4th year.
I believe this is because the bonus is separate from salary and assigned by date at the time of signing. Basically 3 scheduled guaranteed payments regardless of what else happens, not an annual payment of an indeterminate length.

To my understanding anyway. Cmiiw.