SSR Eidolon takes priority over the SSR weapon if you don't even use the element of the weapon, while you can still use the Eidolon without issue. If you're using a full SSR eidolon and weapon set already, then it's all about what gives you more benefits.

You don't need to get both gacha tickets but you should at least grab 1 of them, the first one is cheap and shouldn't be an issue.

SR weapons in shops only need to be gotten if it's an ATK↑ skill and weapon type you know you won't be able to get in the future from events, such as the SR Fire sword. That or if you want them for collection purposes, get 1 of each to at least fill your library. Aside from that, not much reason to get them, especially not the HP weapons unless you literally have nothing better to use them on, you can at least use them for leveling up skill for better weapons which you can just do with raid boss weapons.