We recently re-added Commit/Memory to support Narset/Hullbreacher/Leovold combos. Our players specifically requested this card because it interacts outside of the combo, so you'd likely play it in a deck with only 1 of these pieces, particularly if you have tutor/dig capabilities. As others have mentioned, Echo would likely only get played in Storm.
1 - TBH I'm not a fan of Char. I think (and would like) my red decks to be more tempo based than burn based
- Mizzium Mortars can count as a sweeper, especially if you're cube is filled with tokens. Spearing Spear variants usually don't make it into red based combo decks, but Mizzium Mortars + Magma Jet (scry) usually do. Its also okay in Rabble red decks and control decks. Its one of those cards that is borderline good enough for every red deck and also helps slower decks with the density of answers to strategy, but is never excellent. I would play this if having answers/ density of red removal is important for you
- Soul Sear does a fine impression of Hero's Downfall. 4 damage vs 5 damage out of a red deck is a huge difference - See GloryBringer vs Thundermaw Hellkite debate. Its a fine card for red decks that rely on Koth, Chandra and Hazoret to win the game than the traditional sling decks. Its also pretty good card in control decks - but with more planeswalker removal printed in Black/ White this loses some luster.
I personally have Magmatic Sinkhole and I would love to have a second red hero's downfall style effect ...
I think both are fine options, but I would look at your cube's removal density to fit your needs
2 - I second steve_man on this
3 - Ulamog > Emrakul > Triplicate Titan. Without Tinker, Daretti, Welder, Triplicate Titan loses a lot of its luster. Ulamog the Ceaseless hunger is can be channeled, reanimated, sneak attacked.
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I'm actively maintaining a comprehensive article to help explain to new cube players how some complex vintage level cards work in a cube environment.