Advice

Grail your favorites.
No grailing isn't necessary, I don't play the JP version but even talking to guys who do, I've never heard of a need to grail outside of wanting to solo something with that servant. its mosty for love/bragging rights.

Outside of "if theres a servant you love/waifu, grail them to 100", My advice if you want to get the most out of the grails is if a servant is at level 90, leave them like that. most servants gain about 1000 in attack/hp for every ten levels grailed, level 90 > 100 costs you 5 grails and gets that servant 1000 atk/hp. For the same amount of grails you could grail a 3 star to 90 and give them about 2000 in each stat or grail 2 4stars to 90 and give them 1000 to both stats each. So its more economical to make 3 and 4 star servants you like better than making the 5 star you like, who probably already did their job fine, even better.

Grailing your favorites is the usual answer...

But as someone who doesn't have waifus/husbandos and plays the game for its strategy aspects, I've been considering this from a pure gameplay perspective.

Grailing for HP to me isn't a very effective investment. Even at 1k for 2 grails up to L90, that seems rather expensive for what you get out of it. Sure, there are times that this makes all the difference, but IMHO I'd first spend my grails elsewhere. Grailing for HP after L90 is completely not worth it.

When considering who to grail for ATK, one of the general considerations is that classes that have an ATK weight modifier above 1 are better to grail than those below 1. Effectively, that 1000 ATK you get from 2 grails becomes 1100 on a berserker, but only 900 on a caster. I'd also say that unless it's a crit servant, Buster-centric ones enhance this effect even more, as Buster cards "turn more" of the ATK into damage.

Grailing to L100 even for ATK is probably not worth it, with maybe the exception of Herc with his bond CE who you set up as the "last man standing". But even so, it's a pretty big expense for a seldomly used possibility.

Yea like Larry said, grail who you like. The game is designed so you could beat it with free servants. For team comps, try to stick to leveling one type of team at first (buster, arts or quick) and go from there. As for tier lists, they are a good guide as to who is the "best", but its a single player game so its not too too important.

As they’ve pretty much all just said, grail the ones ya like.

As for question 2, not sure from what you wrote if you are actively trying to avoid friend support. Also looks like you are looking to build your teams all Sabers, lancers etc. Sorry if I am misreading. You ideally would want to focus on building around your one main damage dealer or spamming NPs depending on your line ups. For archers, you’d want to pair Altria with either Tamamo or Caster Gil to boost her Arts damage and do high NP damage. You could use Robin with Tama to help spam his NP. For any class you want to build arts/buster/quick comps based around your best damage dealer and the enemies your up against. Usually that doesn’t involve all servants of the same class. But I apologize if I was just misinterpreting what you wrote.

I pay little attention to the tier list my self. I actually think it’s pretty accurate and well thought out, but there’s fun to be had in experimenting with different teams and finding uses for the servants on the lower tiers. I definitely have some 4 and 5 tier servants who are staples in my line ups lol.