First of all, that's what this thread is for: good questions like yours. So let me take my time and go through these.
Yes, a follow current contact instruction would be VERY logical. In an unambiguous situation, that would save us a lot of grief as our brain-dead operator keeps switching targets to the closest warship or just the closest enemy vessel, depending on your orders. Heck, he doesn't even do that reliably. Actually, he should be able to follow a current contact while still retaining the ability to advise you of new developments that could change your orders.
Were that idea implemented in the game, we would then be talking about how the sonar operator is so perfect in a convoy situation, able to continue following a single target even when you couldn't pick him out if you were using the sonar yourself. Every silver lining has its cloud, you know, and Ubi could have turned this into another iteration of perfectly plotted visual sightings and plotted passive sonar spikes whose lengths tell you the range.
Now, if you think about it, the MOST confusing situation would be a small convoy of merchants. Why? In a warship convoy, you have many different types of target, all with different sounds... Well, not in SH4 because we only have two sounds: merchant and warship. These are played at a finite variety of speeds to convey slow, medium and fast. That's why we can't do turn counts, which is good, because WWII subs really couldn't convert turn counts to speed either because they didn't have sufficient data on non-warships to do so.
In a merchant convoy, you are more likely to be dealing with half a dozen to a dozen identical merchants with no difference in sound to speak of. They are lined up in columns and rows in such a way that even when you are close, the contacts are continually crossing, giving you a random choice, when the single contact divides in two, of which one is your original target. However, let me suggest that you try tracking the closest target in front or at the rear of the convoy. This way, every time contacts cross, when they divide, you automatically pick the front or the back one! I think this should work even in the game as we have it now, so long as the AoB is less than 90º. After 90º, the lead target would always lag the crossing target and the last target would always lead.
That should be no problem, as your plot would show you the parallax situation created by the AoB and you would know what to expect. Is anyone else following my reasoning or am I being way too abstract here?
Along with grabbing TMOkeys (which you will like much better than the stock keyboard layout, once you are used to it. I bet you'll use it even if you stick with the stock game), you might also grab TMOplot, which will let you play with the TMO plotting system. TMO tosses out the ship silhouettes, the target ID text, and the friend/foe/neutral colors. That's why I only found out I sank the USS Essex after I returned to port. That's why I fired the torpedo. I had no silhouette telling me it was an aircraft carrier and it wasn't green. Result: bustin' rocks! What's not to like?
As far as the passive sonar spikes telling you the position of the target, why plot those points? I just note that particular target seems louder than the rest and leave it at that. Certainly a competent sonar operator would tell you that. He would also tell you that local listening conditions could mean that the loudest contact is not really the closest, but we are talking about an imperfect game here. I treat it just like I treat visual sightings. Plotting positions and making speed and course measurements are a no-no for those two categories of info in SH4.
I look at having sonar spikes extended to the edge of the screen as the same thing as turning map updates off: the solution is worse than the problem when a little discipline in not taking measurements from those positions is all that is needed. After all, in real life that competent sonar operator is making decisions about what information to pass on to you as commander based on his assessment of which are the closest targets of concern.
I am running JSGME with a Steam installation of Left 4 Dead (amazing how all those other games use the brute force method of mod installation. Dummies!) and it works flawlessly. I've also used it with a non-Steam installation of Grand Theft Auto: San Andreas and it worked marvelously there too.