Ozy, Medb, and Surf’s Up! Mordred

memerlin is coming buster crit teams gonna be op if I were you I would bet on ozy

You are pretty much spot on in your thinking. Mordred without Nero Bride and Tamamo is just dead in the water. Just use Saberlot with hans and mash and youre honestly golden.
Making Summer Mordred work needs at least NP2 , and the mentioned trio to make her comparable to a standard buster crit setup with Jalter, Raikou and Ozy that require far less 5 stars since hans is F2Pand waver is not limited. If merlin is added then rider mordred is just blown out of the water.

She is fun to use but needs too much to be Viable in harder fights. Youre just making things harder for yourself as a new player if you start investing heavily into her early on

Her gimmick is basically Spam Np for the charge reduction which is 80% chance I believe. Pair her with tamamo who also has charge reduction and skill cooldown and you can stall fights forever. Nero is just there to boost her damage and NP gain so the chain never stops

Get medb to the 70 at least. Prisma illya is the thot event and you know what medb be like "If it breathes it a thot"

For the first question this is a bit hard since Mebd does give extra 50% damage in the event whereas you could use Ozy, but he doesn't give off extra damage and couldn't receive extra damage with the event center which it hard to decide since Mebd is going to be useless after the event.
The question to answer this is that do you have more female servants? If that is so then I suggest raising her just levelling her to at least 50-60 just to help out and if you have decent levels and considering 200 rider embers you could easily do that. Then see if you could just use her like that if not then raise her level with the embers in the shop.
Still take this with a grain of salt.
Most of the bosses are casters who are HP tanks like those monsters from Camelot though I haven't heard of gimmicks, but their health is high and I'm probably going to rely on a Medb Drake no matter how much of a joke it is. This is because of the double buffs of normal damage and buster performance on the event CE, and simply think of the single target riders we have? Ozy is male, Ridertoki is male, Anne and Mary has a quick NP not arts or buster which is needed for the performance boost, Ushi has a quick NP.
Things would be good with quetz around, but she isn't, sadly.
Still after hearing that I would just let you decide.

About rider mordred. Ozy's buster crit is about hitting like a truck with an NPBB chain with a crit while Mordred is quite simple. Using her arts buff and a high hit NP and Arts chains to regain here NP which wouldn't work without her first skill. Remember why Tamamo was there? Quite simple she increases the effectivity of arts teams. Damage with arts performance boosts, cooldowns and heals. Two of them important for rider Mordred to spam so I suggest not going for that route. Arts teams always need a support to be extremely good and running three damage dealers would frankly wouldn't work that much as they lack supports to bump their damage that buster crit usually has reduce cooldowns on their skills that make arts servants great.
I suggest trying to find a friend who has Tamamo in the list and removing Dadcelot when using Rider mordred or Rider mordred when using Dadcelot since there might be a chance that Rider mordred would steal stars specially when his skills aren't active. In anyway that team might work, but it wouldnt be great and would just be a generic NP spamming team without that much damage.
Sorry for the long reply.

"It looks like there will be minimal time (if any) between summer and Prisma Causeway"

I really, really hope so!
No time for grinding as part II starts. Need to level up my females Carmilla, Medea, Surf-Mor and Anne & Mary to 60 at absolute minimun.