To Burn or Not to Burn

Orion I would keep. Although archer Artoria is, in general a better pick for the single target Damage role, Orion's bulk and anti-male damage boost might prove useful in a challenge quest. If you do choose to burn, just make sure to bond enough to do interlude 1 and 3 since they give Quartz.

For Tamamo, the NP2 actually does 500 more healing... which is still a largely insignificant amount. However, one aspect worth considering is the flexibility of a 200% NP bar. Being able to go past 100% allows a servant a degree of protection against NP drain. If a boss uses a 30% NP drain on an NP2 Tamamo who's at 130%, versus one capped at 100, could be impactful. Whether that small benefit is worthwhile depends a lot on how commonly you use Tamamo, and personal preference.

Definitely keep Orion. He's not as consistent as Archertoria but he's still a staple for arts drain strats. A good fallback if Archertoria falls.

For Tamamo, yeah you're right, having her at NP2 is not really a huge boon but overcharge is never a bad thing and its pretty rare to get a 5-star, even if its a duplicate. Anniversary blonde is a good MC but may not be worth throwing away an NP2 SSR.

If you're asking to choose whether to keep Orion vs Tamamo, then most people would say to keep Orion, as she is certainly useful for her Anti-Male capability.
However, the question is weighing the value of whether it is worth getting Tamamo to NP2 or burning for the Rare Mana Prisms to unlock the last CE you want from the shop.
In my opinion, unless you're a whale, and have tons of money to throw away, treasure every 5* you get, and increase NP levels whenever you pull a duplicate.
My opinion seems to be the unpopular one here, but I'll explain my reasoning.
Tamamo at NP2 heals an extra 500. While 500 hp doesn't seem like a lot, it is still 500 more hp the enemy has to kill, and could mean the difference between a Servant getting offed or surviving for one more turn. In addition, she charges NP bars an extra 6.25%, which while it also isn't a lot, could still be the difference needed when you're one attack away from a full NP charge and a quick card is pulled instead of an arts.
Every little bit helps, especially if you're using her as a Support for challenge quests, and I guarantee that you'll appreciate just that little bit more HP and NP gain.
The benefits of having a NP2 Support Caster vs NP1 outweighs having some Rare Prism CE just a little earlier in my honest opinion.
From the looks of it, and I could be wrong, but it looks like you roll Story Gatcha a lot. I spent a couple hundred SQ on the Story Gatcha, and also pulled Orion and Tamamo out of it.
Even if you dont' roll gatcha often, shouldn't take long for you to get a 4* Servant that you can't really use to burn for that last Rare Mana Prism, especially if you plan on rolling big during the Summer gatchas. It means that you'll have to wait a few more months, but really, Getting a 5* top-rated Support Servant to NP2 is MUCH MUCH better than getting a Rare Prism CE just a few months earlier.
It's important to think about what you're aiming for in the long term for this game rather than reaping the small things in the immediate term, as you never know what types of difficult challenge quests FGO might throw at us, and when it comes around, you'll really appreciate that extra 500 HP heal when you're grinding a Boss node.