Good point. I do wonder how accurate it is, quick google suggest that its effective range is 2,700 meters which I am skeptical about that, but you could probably improve its performance with some modern modifications.
For a machine gun, "effective range" is the distance to which a bullet can kill. You won't be pulling pinpoint accuracy at such ranges, but the only time you'll be firing at such extensive distances is to break up/harass clusters of men, a task for which precision is not required. This is the same reason that late 19th/early 20th century rifles had sights that went out to ludicrous distances - that was for an entire unit to lay down area fire.
At more reasonable ranges, the Maxim is quite accurate, probably more so than most machine guns because it is such a stable platform (due to being incredibly heavy)
How about T-54/55, given some modifications can a quantity of these cheap and reliable tanks be useful on this battlefield?
In theory, of course they can. Ukraine was given just under thirty Slovenian M-55S tanks. These are T-55s refitted with a NATO 105mm gun, reactive armor, significantly improved fire control (an electronic computer, gun stabilizer, and laser rangefinder), and massive upgrades to the optics. Thus configured, it becomes almost equal to a mid-range T-72 in capability, the weak gun and armor being offset by more advanced fire control and optics. Practically, the only reason Slovenia was able to do this is that they were already operating these tanks domestically, and were in the process of phasing them out in favor of similarly upgraded T-72s. Developing such an upgrade program from nothing is practically impossible, and even if Russia has plans for such, it is very unlikely they have the sophisticated electronics you need to make useful improvements to combat capability.
A mostly stock T-54/55 is very unlikely to be useful. The 100mm gun isn't terrible, at least for flanking shots or against non-tank vehicles, but there's effectively zero stabilization (meaning the tank has to stop to shoot, and stop for several seconds to get the gun sway under control - most later Soviet tanks can fire from short stops or low speed movement, and modern NATO tanks retain full accuracy while moving at full speed), rangefinding is effectively nonexistant, and the optics are just glass. No night vision, no thermals. The armor is also marginal - 200mm turret/120mm hull of plain steel isn't that great, and that's the frontal armor. Any tank gun Ukraine has access to (be it the 125mm guns on their Soviet era tanks, the NATO 105mm, French 105s, or NATO 120mm) won't even notice. Neither will any dedicated anti-tank infantry weapon - even a humble RPG-7 or AT4 (hell, if they dug up a M20 Super Bazooka somewhere) will blast right through. Even some of the bigger autocannon on some of the IFVs being donated will go through, though the M2 Bradley's gun won't do the job (they have TOW missiles for that). From the sides, most autocannon would be able to penetrate. A T-54 is no match for a T-72, and a Leopard 2 or Abrams (note that Abrams deliveries have been hastened, probably enough to get them in country this year) would have no concerns except ammunition supply.
Using them as infantry support is effectively suicide, because literally any anti-armor rocket or recoilless rifle round will take one out, and those have been very generously seeded at low levels in Ukrainian forces. Dug in as fortifications is also not all that great an option, because Ukraine has precision-guided munitions that have been used to extremely great effect - the roof armor on a T-54 is thin enough that even the non-explosive area-attack munitions from a GMLRS rocket might punch through. Training is also dubious - these tanks are so obsolete that you'd basically have to retrain from scratch in a newer model. They might be useful for flank patrols against aggressive raids with light armor, or to bulk out an attack spearheaded by real tanks (where their role would be to divide defensive fire and provide additional suppression), but that's the likely extent. The old adage that "any tank > no tank" is a good rule of thumb, but there's a point where a vehicle doesn't even count as a tank on the modern battlefield, and the T-54 is pretty close to that line.