Not guarding Earth, its guarding the relay in.
Close enough, Arcturus prime directly connects to the Charon relay, which is the Earth relay. Intrarelay travel is nearly instantaneous, and the journey from Charon to Earth, going at the rate of 12 lightyears/day (the given "typical speed" for FTL), will take less than 10 seconds.
The longest part of the journey, by far, is mobilizing the ships.
The Collectors, as agents of the Reapers, have at the very least the ability to work with the Relays to change where they head,
Source please.
this is mentioned as rumors several times in game,
Which rumors are they exactly.
and partially confirmed later in the game, as they only have one Relay in their system, yet can appear anywhere and any time.
Their relay is a one way trip to Omega, and there's another relay
in Omega which connects to the rest of the system. Appear anywhere at any time? Why do you say that? There are no FTL sensors, the whole appearing out of FTL thing is standard fare.
Even scarier, they might have access to the same thing Sovereign did in that they can shut down a Relay
When the hell did Sovereign shut down a relay? The relay network is not on remote access to all reapers otherwise the plot to Mass Effect 1 would have been redundant in the extreme. The Citadel controls the whole network, not individual reapers. Neither reapers nor collectors have demonstrated the ability to do any of the things you're stating.
Like all frigates, the Normandy is equipped with GARDIAN point defense lasers, kinetic barriers, and a spinal mass accelerator cannon. It may also be equipped with advanced Javelin dual disruptor torpedoes.
Digging up quotes from the SR1 when the discussion is about the SR2? classy.
Here is some info from the SR2, you know, meaning it's relevant:
Personal upgrades are numerous: the airframe could support additional armor and an axial mass accelerator,
In case you're not in the know, "axial mass accelerators" are ones strapped to the middle aiming forwards, where the Thanix gun is.
The SR1 had an Axial mass accelerator, but the SR2 didn't, implying that if it had any at all, they were not mounted on the centre. If they weren't mounted on the centre, then they were on the wings, which fits perfectly with what we see.
You think they were
disruptor torpedos??????
First of all, the SR2 doesn't even have them. They have Javelin launchers, which are described as follows:
The torpedoes are fired on converging trajectories, and detonate in a precisely timed sequence that allows the dark energy emitted by their warheads to resonate. This magnifies the resulting space-time warp effects.

The fact that this is not a disruptor or javelin torpedo should be incredibly obvious from the fact that it has no warhead anywhere (which disruptor torpedoes had in the ME1 cutscene....) and looks pretty much identical to the handheld mass accelerator weapons being fired, with the blue glowy trail.
But in addition, those two so called "torpedos" in the image are not only not traveling on convergent trajectories, they're moving
away from each other.
I repeat: We saw disruptor torps in ME1, they look nothing like that.