According to the sensors range chart you are able to discern the basic outline at long range. At that range officially you can only detect that EM is going on, not what it is. Thermally you can detect the overall heat of the thing. Active RADAR and LIDAR will give you the basic outline (small and missiley not big and spaceshippy.
I didn't think Traveller had magic heat management, it is one of the few things it does need to shed into space... cold cold space. Where a warmish thingy will stick out like a sore thumb and a smallish warm thingy will have a hard job to make itself look big enough. Thermal imaging target identification is not the same as an IR seeking sensor.
For you to deploy your countermeasure you will have had to spot the attacker so you must already be in those ranges as currently all sensors have the same range and the super sensors are Densitometer and NAS which both require you to be much closer.
I am looking at those complex weapons you identified and despite their clever RADAR gizmos ain't one of them looking like an F117 or a Stratofortress to me with my visual sensors. You might fool an automated RADAR guided air defence system to fire on you as the nearest suitable target, but you won't fool an operator looking at an image from a RADAR directed telescope with electronic optical enhancement. And the RADAR will find you because you decided to say "hey look at me I'm a Stratofortress". They are also designed to be launched from stand-off ranges and behave like the relevant aircraft, not do something bizarre like suddenly split into four aircraft.
You might create useful targets to distract the pirates own volley of missiles (and that is a definitely a good thing) but you are not guaranteed to fool a sensor op (though you might make his job harder - so a negative DMs). If a ship trace suddenly breaks into four traces he knows something is up and will be looking for anomalies.
SEAD is a very specialised field with very specific aims and targets in mind much of it not suitable for public dissemination (but it is obviously the theoretically impressive volume of Soviet air defence systems and their skilled operators). Since Ukraine a lot of that design theory will likely change as the value of "many" has been devalued somewhat.
Active sensors can be spoofed into making the receiver see what they want to be seen. Radar requires a signal return from an object for any detection. That signal can be jammed so that they do not get an accurate read. An object can increase/decrease it's signature to make it so that the return signal is not accurate at all. That's the point of a decoy - to make the viewer see something that really isn't there.
Lidar can be easily jammed with a laser directed at the sensor. By sending back infared pulses along the same frequency as the emitter you will blind the Lidar, or at least give it junk data to interpret.
The trick to any decoy working is to be able to deploy it so that the enemy can't make sense of which is the real target and which is the decoy target. This gets far more complicated as the size of the ship gets larger - there has to be a corresponding size increase to the decoy in order to generate the necessary false target info. Plus your decoy has to be programmed to emit the same electromagnetic/thermal signature as well as having ample power to maintain that spoofing for a period of time. This means a tiny little missile may not have the ability to act as a decoy except at longer ranges where sensors are not able to burn through the spoofing. Electronic warfare is often about who has more power to burn through the other side. In the real world ground radar installations have far more power than airborne (or seaborne) ones, however they also are trying to cover a very large area whereas the aerial ones only have to cover where they are. And there is distance as well, as the closer you get the less effective your spoofing/jamming is.
F117 utilized a distinct aerostructure to minimize and scatter radar waves to achieve low radar observability (trivia note for those who played the Microprose F-19 stealth fighter game - nobody expected just how butt-ugly the F-117 was gonna be in real life as compared to the computer game aircraft model). It also had special radar-absorbing paint to help reduce it's radar signature. The BUFF, on the other hand, looks like a giant barn on radar due to it's size and lack of stealthy design (which wasn't an issue way back when it was first designed). Newer generations of stealth aircraft also utilize specialized coatings to minimize radar returns as well as reducing the radar return of their structure. IR detection is much harder to defeat since you can't really hide it, though some aircraft minimize their forward and side signatures by reducing how much of the exhaust area is visible.
Whether or not you will fool a sensor operator greatly depends on the skill of the two operators (you can have a defensive one as well) involved AND just how good the equipment is. Mainline Imperial military tech is supposed to be TL-15, and with an average of TL-12 across the Imperium, civilian sensors will have a very hard time against true military gear. One would expect colonial / secondary ships to be TL-14, with system defense and local forces being TL-13/12, erring on the side of TL-13 simply because it's pure military stuff and not civilian. Pirates would/should have mostly civilian gear to play with since they won't normally have access to the gear or spares to maintain it - unless they are privateers or someone that is state-supported.
SEAD is indeed a specialized arena, or rather it used to be more specialized back during the Cold War. Today you see militaries that have invested less and less in dedicated platforms for it. Look at the decrease in the overall numbers of wild weasel and other dedicated EW aircraft. Instead they are moving more and more to automated jamming pods rather than actual operators. The USN is wanting to cut their Growler fleet but so far Congress has not authorized it. The US Air Force has gotten rid of most of its aircraft as well and moved on to pods (EF-111 used to be their premier airborne platform). Russian EW in the Ukraine warfare seems to be mostly ground-based, as they have been hesitant to commit modern air platforms to the war (not to mention the Ukranians have shot down a few of their highly prized air assets, and not the ground attack trash either).
From a pure gaming perspective it really comes down to how complicated do you want to make things? At short range most decoy efforts should fail since the enemy is simply going to be too close for it to be able to be effectively deployed. By effective I mean it's able to be deployed undetected so that the enemy simply ignores the decoy. At medium to longer ranges it starts getting better chances of being effective, and at very long range the advantage should lie with the defender - all other things being equal. And if you are going to go down this path then you really have to start looking at just how decoys work and how much someone is going to invest into them for them to actually BE effective. That means a significant expenditure in electronics and power generation - and that gets out of the realm of civilian and into the pure military. And that's gonna be beyond the average free trader or pirate ship.
In the case of a thermal sensor in a missile that is just using it to home in then all you need is a sufficient temperature difference between background and the object.
A Mechantman that’s firing before they get fired on is going to be the target of any patrol ship in the area as well as getting tagged as a pirate if some survive with their transponder data. Which is another reason to have lasers and not missiles, the lasers can be used defensively as well as offensively, missiles even your sensor missile would always be perceived as an offensive weapon.
