Do these cancel each other out?

Hrid cancels out Ephraims follow up and does one of his own. So no. But because Ephraim is faster he's gonna double him anyway. So its a cancel out but its still in Ephraim's favor theoritically because he still doubles.

L!Ephraim's follow-up granted by the weapon and Hrid's weapon denying the follow-up should cancel out and result in a Spd check for Ephraim. (Hrid's ability to make the guaranteed follow-up should still remain since Ephraim doesn't have any abilities to stop it.)

Nothing about Flame Siegmund denies follow-ups, so if both were to apply, Gjoll would cancel out Flame Siegmund's follow up, and Hrid would gain its follow up effect. However, similar to Wary Fighter, if Gjoll's denial is in effect and the affected unit has multiple means of causing follow ups (like a weapon effect and by meeting the Speed threshold), they are still able to perform a follow up.