Great to hear you like the Alt.
Yes you're dead right on the melanoiden malt. I use a little in lagers when I am not doing a decoction. I am planning a rebrew of the Helles without the melanoiden and with a decoction. A decoction mash is certainly a traditional approach to making a Helles. It's fun to emulate these things I think.
Recent similar lager examples include two
German Pilsners, one with melanoiden (similar %) and an infusion mash. One without melanoiden and a decoction mash. Both were very similar in appearance and malt flavor. I really enjoyed both.
Pils with Melanoiden
Pils without Melanoiden
On the yeast question, prior to this Helles I brewed one with WLP029 (the Kolsch yeast). I know it is a popular choice but I am not a fan of this yeast for lagers as the beer (at least to me) did not taste like a light lager should taste.
Despite fermenting the beer in the mid 50's with WLP029 there was a very clear floral component to the beer from the yeast. Not at all unpleasant but not what I want in a lager. Just tasted like a Kolsch.
In my view if you want a lager to taste like a lager you need a lager yeast. Particularly in a Helles where the subtle yeast derived flavors will not be hidden. The two Helles beers (lager and hybrid) were very dissimilar.
To each their own of course. Best of luck with whatever you decide to brew.