To me Helly looked like the rest of the team in this series, mentally and physically exhausted. I attribute it to fatigue and it finally catching up to him and not confidence or puck luck. I said at the start of the year that I was thrilled about the Mason/Helly tandom as it would get us in the playoffs. Mason getting hurt gave Helly the reigns and although a I liked that he played lots and played well he did play too much at times, and during the season I kept saying that I hoped Mason returned soon as we were gonna need him to play some games the last few months. It's too bad he never did come healthy for a long stretch as I think Mason can be good, and they rode Helly a little too much, and it didn't affect him, IMO of course, until the latter part of the playoffs.
The last 7-8 games or so he wasn't tracking pucks (reacting) as sharply as he did all year and was making mental mistakes whether it be handling the puck or getting out smarted by the opposing player. His rebound control also slipped and I found he was a little too deep in the crease. I don't think any of this happens on purpose but sometimes mental fatigue affects your game just that slight bit and that's all it takes in the NHL to get outsmarted by shooters (I will speak more to this below), he wasn't horrible but played average and that is the reason I think why.
Interesting you bring up the debate from last summer, I saw a brief clip of Helly on CBC at the start of one of the Preds games where Helly said this:
"No matter what happens, at the end of the day I believe in myself. Confidence is a big thing, you have to be able to trust yourself and sometimes your making reactions not based on what you think you're gonna do but based on something happening and all of a sudden your glove pops out, that's what confidence does, it give you that half a second of patience that maybe forces the shooter to make the mistake. Sooner or later you're just so confident in the net and shooters can see that presence"
I had a healthy debate last summer that confidence was all Helly was missing and that he needed that last year to learn and adjust to shooters. (Last year he was second guessing, this year he was reacting, confidence sure helps) What he said above is exactly what I said last summer and how it's such a fine line and what a split second means at the NHL level. When I saw this video with him talking about it confirmed it for me, that he now had it this season and was outsmarting players. (Note: this happened less this series, I'll say why below)
Now to close, and where I'm going with this, is I don't see confidence as an issue down the stretch but fatigue, but the end result can be the same, being a split second behind makes all the difference in the world, whether playing the puck behind the net or playing in the crease, he just didn't look as sharp and quick with his decisions and that split second difference means the puck is in your net. That's what it looked like to me, because he didn't just all of a sudden forget how to play goal, just like the Jets didn't all of a sudden forget how to skate, they looked tired. Sometimes it happens, and I think all the accumulated minutes he played finally took it's toll. Now this is all my opinion of course and nothing more than that, but just like the Jets he looked tired mentally and physically, but just like the Jets Helly will bounce back next year and be fine.