I don’t think that Erza and Jellal would engage during the timeskip. Jellal, from the last time we saw him, still thought that Erza and he was walking down the different paths in life. I can’t see why all of a sudden he would change his mind and think that he and Erza could be together. I totally agree with you too that if they engaged, it would make his entire talk of encouragement with Erza in Ch. 416 lose meaning.
As for Erza, I don’t think she would propose to Jellal lol. In the last 6 months, she might have work with Jellal for sometimes on investigating Avartar but that was all in my view. It didn’t look like something that significant to the point that would change their current status. She was acting very neutral when she mentioned Jellal. If she had engaged with him, I also think that she would have given hint too.
Personally, I want them to be together after Jellal finally forgives himself and understands that he can walk the same path as Erza does.
But I have to say that Erza hiding engagement right under her gaunlet is a creative idea.