There can be some underlying causes contributing to recurring Tube leakages, already present but now surfacing more frequently. What sort of tube damages have you observed in terms of inspection findings? And what exactly the tube leaks locations are?

I would say the heat input stands out.
Maintain the interpass temperature at 100-150 C to make sure those harmful intermetallic phases do not form.

That is an important question that what sort of controls can be applied with making the process workflow in the system.
In my opinion, there is no issue in utilizing existing systems to support PSSR as well with the checklists embedded within. Reason being that everything is there in the system ...

1. Have these heat exchangers facing such tube leaks for the 1st time?
2. Any recent operational changes happened which might have gone unnoticed?
3. What about the heat load and other variables as you experience these leaks?

What are the minimum requirements in preparation of corrosion coupons?
If not installed initially, can they be considered to be installed? What should be the controlling /monitoring protocols?

Identifying metal dusting from other damage mechanisms including carburization, oxidation, erosion corrosion, sulfidation and flow accelerated corrosion can be tricky.
What are the leads here to get to the correct identification?

What a useful explanation to remember.

I wonder how application of RBI software to take of inspection of PSV helps.
What are the data leads provided as mandatory and what are the expected outcomes?

There is a rotated square 45 deg layout as well. This arrangement:
It helps in:
Maintains cleaning lanes.
Provides somewhat better heat transfer than 90° square pitch.
Often considered a compromise between triangular and square layouts.

Practically, ben has listed all the potential causes. However, when bellows failures suddenly increase across several PSVs, the most common root causes are typically:
Increased backpressure in the flare/relief system.
Process pressure cycling causing fatigue.
Corrosion or stress corrosion cracking ...