IMO the arm in guillotine should rarely be committed to. It's a low percentage move. It's great to control your opponents head, not so much to finish them. Me personally, I swear by no arm guillotines. If you look up fight footage of successful guillotine finishes, I can practically guarantee that at least 80% of them don't have the arm in. Marcelo's not an arm in guy either. Jones finished Bader witout the arm in. It's just 100x easier to finish a guillotine without the arm and you can finish with less torque.
I mean yeah, it's worked before. I've even seen guys go to sleep from arm in guillotines, but IMO, it's such a low percentage move that it you shouldn't burn gas committing to it unless you really feel your forearm deeeeep under his chin or your arms are "v"d around both carotid arteries. 9/10 you should use it to control..and if you have it tight, adjust and take the arm out and go for a no arm guillotine.
That's my 2 cents.