Q: Is it possible to cheat with Rollem?

To the best of my knowledge: No.

It is not possible to convince Rollem to roll higher/lower die values for you. However, as with all PRNGs, if you can figure out the internal state it's possible to predict future numbers. As of Late May 2021, Rollem is using a CSPRNG, which makes predicting values extremely difficult (and the topic of active research in the security field).

Prior to Late May 2021, Rollem used Node's PRNG which, while probably more random than actual dice, is not resistant to a determined attacker.

Here is the line in Rollem's parser that rolls the dice.