Is there any way a griefer could overcome my 5-million-block quarantine?

While it may or may not be feasible for a player to make the lengthy trek to your area, the simple fact of the matter is, they don't have to, to grief you. Destroying your stuff isn't the only way to grief your server

As has already been mentioned in the comments, there are things they could do to slow the whole server down; massive animal farms are an easy way, as would be a large redstone contraption that mostly just triggers a vast amount of block updates every tick. And the worst part is, if someone does try to make something like this, you'll have to actually remove it somehow; just banning the player responsible will leave the source of the lag lying around, with its chunks unloaded, and once another new player shows up and loads those chunks, the griefing will be happening all over again.

Admittedly, griefers who will do things like this are rarer than people who will just wander over to your place and trash it. And there are ways to prevent it, and/or deal with it after the fact.

• You can use something like MCEdit to directly clean up the situation, even if the server is utterly crippled.
• You can increase the spawn-protection value in your server.properties file. Crank it up enough, and all new players will have to wander far enough away before they can do anything that a brand new player won't be loading potentially-compromised chunks.
• You could also turn this around; leave the default spawn at (0,0), crank up the spawn protection enough to cover your place and a large surrounding area, and non-op players won't be able to damage things inside that radius.
• You could even go more draconian, and create a wall around the default spawn, inside the spawn protection radius, so new players can't get out of it, then require an op to teleport all new players out after having a brief chat. Most griefers will give up quickly when they can't do anything, but you'll probably also lose a lot of good players if there isn't an op on pretty constantly to greet new people.

Ultimately, while isolating yourself from new players might be useful in preventing griefing, it's not a solution on its own. At the very least, you'd be leaving new players at the mercy of the same griefers you're trying to keep out, which could very well end up discouraging the exact sort of players you're hoping to keep around.

The fastest way to teleport in vanilla Minecraft is though Nether.
From the Minecraft Wiki:

Locations in The Nether correlate to Overworld coordinates, but Nether distances are scaled down by a ratio of 8:1. Therefore, traveling one block in the Nether means traveling eight blocks in the Overworld. This fact can be used as an Overworld travel shortcut, though the Nether is significantly more hazardous, with more complex terrain, making navigation difficult.

With the simple calculation of 5000000 / 8. One would need to cross ~625000 blocks. Other sources from the wiki about sprinting and walking indicate:

Sprinting is a method of transportation that allows the player to move faster, around 5.6 meters/second. This is about 1.3x faster than the normal walking speed of around 4.3 m/s.

Now again with math if someone ran ~625000 blocks (Straight with no interruptions) it would take ~111607 seconds or ~31 hours.

I don't think anyone will waste ~31 hours running to your house. Other than the neither I know of no other ways to bypass distance without the use of commands and or command blocks. Hope this helps.

As stated, the quickest way to travel is through the Nether, but it is not on foot.

The fastest horse, according to the wiki, moves at about 14.5 blocks/second. In the Nether, where players only have to travel an 8th of the distance as the overworld, a player traveling straight to 0,0 from an overworld point 5 million blocks away could do so in no less than 12 hours.

As mentioned in the comments, this is a long time, and travel would obviously not be this quick, but it's doable over several days, especially if the user knew where they were headed (IE, 0,0)