Err, you realize altering the Senshi's power levels as you wish would require a rewrite of Chapter 3, if not the entire story.
With the pincer attack in the alley, if the Senshi are more at canon levels, here's what likely happens:
Fate tries to block, being a mere mortal, this is a bad idea. Both go through her shields, surely I don't need to explain the end result? Don't think I need to... But, of course, being that you'd rather their power levels be somewhat closer to the manga, where, Mars turned Jadeite into a charred skeleton right after awakening. You know how hot fire's got to be to do that? Me neither, but I expect it to be pretty goddamn hot! That plus one year and six months, her fire's likely hot enough that that by the time it get's to Fate's shield, the heat has already killed her, and hopefully that fool Mau who's idea this was, convention is a bitch most movie/tv script writers seem to forget exists. Anyone who doesn't know what it is should read the final scene in Chapter 18 of Naruto Genkyouien.
Perhaps Fate realizes the dangers of Fire Soul before it closes in enough to barbaque her and strengthens her shield enough that it protects her from both the fire and it's heat. But she still has Supreme Thunder to deal with, and I kind of expect it to have barrier peircing qualities by nature. Perhaps Fate expected her own mana conversion affinity to help negate enemy lightning damage. Unfortunately, the enemy spellcaster is a biologically immortal psuedo-divine being who is, more or less, the avatar of the planet Jupiter. Even if her mana
conversion affinity would protect against lightning damage, which I don't think it does, it will not be helping her in this ambush. Lightning strikes have so many wonderful effects (That's sarcasm, if you couldn't tell), <a href='http://science.nasa.gov/science-news/science-at-nasa/1999/essd18jun99_1/' target='_blank' rel='nofollow'>take a look at the bottom of this page</a> and pity Fate if some of this was to happen to her. Doesn't really matter if she blocks Fire Soul, as that shield drops when Supreme Thunder hits, either way, she's dead. If she'd flown out the moment they cast she'd have likely survived, but choosing to block instead gets her killed.
Now it can go multiple ways... But either way, for this story, Luna, Mars, and Jupiter just made it onto TVtropes' Fanworks <a href='http://tvtropes.org/pmwiki/pmwiki.php/Monster/FanWorks' target='_blank' rel='nofollow'>Complete Monster</a> page on assumed information.
Idealistic way, and most likely way considering the shows/manga being dealt with, Nanoha gets to watch two unknown mages murder her closest friend. At which point horror and grief cause the manifestion of the Maboroshi no Ginzuishou, with which she raises Fate and Arf. Here starts the very necessary, very violent beating of Luna, Mars, and Jupiter by a very pissed off Sailor Beamspam. After moral is improved by continued beatings, Nanoha and Fate mobilize the TSAB and the Dark Kingdom goes bye bye. Following this, NanoFate return to Midchilda, have the Ginzuishou cataloged and placed wherever Lost Logia go and try to get on with their lives. Wondering whether they took the communicator with
them or not is pointless, as like the Time Gate in that omake, it is likely incapable of connecting to them.
Cynical way, Fate and Arf dead, Nanoha gets to watch this, but is too late to help. She proceeds to get the incantation for Raising Heart correct, and blasts the Senshi with Divine Buster. Unfortunately, like Fate before her, she's using mortal magic, as opposed to the Chaos originated magic all the Senshis' foes use and does as much damage as damage to them as a Hyper Beam on a Ghost-type pokemon. This ineffective attack is followed her trying a Starlight Breaker, which considering the strand it places on her Linker Core, does not get fired. According to SeiyaxUsagi Mamoru has healing powers, which I did not know, but assuming he has them first season, they carry Nanoha to his place, where Mamoru tries to heal her but fails one or more of three reasons, one; because he needs knowledge of what needs to be healed and she's physically fine and like all Earth-based doctors has no knowledge of a Linker Core, two; damage to her Linker Core is so severe that her body is rejecting any and all types of magic, trying to heal just speeds up her death. Number three; Nanoha's still half conscious, no idea what's going on, but knows she's in the custody of those who killed Fate, and is ignoring her instinctive desire to trust them, believing it to be some kind of mind magic, and is actively fighting against all attempts to heal her. A few hours of this and she slips away. Nothing more should need be said...
BAD END
I don't know about the rest of you, but I find I kind of like thinking up ways one little choice could damn a world.