which tense?

It is a grammatical mistake.

The active form of that part of the sentence is
Octopus (Company) has constantly been assuring us (MTRC) that the rules and regulations have been complied with.

The tense in the active voice is present perfect continuous tense. Following the standard formula, the passive voice of this tense is:

We have been being assured by Octopus.

Because of the clumsiness of the expression, present perfect, instead of present perfect continuous, tense is used in passive voice. The sentence should be

We have constantly been assured as with all shareholders by Octopus that ...

There are other mistakes in this statement.
1. as it was, not as it were
This makes the sentence clumsy. The more straightforward way is like this:

The key for the shareholders has always been whether ....

2. wrong use of comma
It should read
We have constantly been assured, as with all shareholders, by Octopus that the rules ....

hung :

we have constantly..............=present simple tense.=have.

being assured....................This is the present participle+pp
= [ be + pp ] root form = passive voice.

我們不斷地有(作出)那百達通的所有股東都被保證being assured.


沒有have + v~ing這條form...只有have+pp=完成式.....be + v~ing=進行式.