Actually, cl. 8.4 is hardly a model of the draftsman's art. It reads thus:The clause 8.4 clearly states what should have been done.
Each party will give the other written notice that:
(a) a condition is unilaterally waived or satisfied on or before its Condition Day. If not, this contract will end after the time indicated for that Condition Day; or
(b) a condition will not be waived or satisfied prior to its Condition Day. This contract will end upon that notice being given.
The clause actually makes no sense. It says "each party" will give written notice. Where, as here, the vendors have stipulated for no conditions, what notice have they to give? Are they somehow to participate in the purchasers' giving of notice?
Also, both (a) and (b) yield the same result. So why have both? If notice of waiver or satisfaction is not given, the contract ends. So then, why even bother having a provision for the K to end by giving notice of refusal to waive?
A more felicitously-worded clause would be:
Each condition contained in this agreement is for the sole benefit of the party indicated, where so indicated and unless each condition is waived or declared fulfilled by written notice given by the benefiting party to the other party on or before the date specified for each condition, this contract will thereupon be terminated and any deposit paid hereunder shall be immediately returned.
Anyway, getting back to what occurred here, while money_talks claims legal expertise here, as does mrbizi, I'll stick with my view that we have here a reasonably arguable case that the purchasers, through their agent, effectively communicated their acceptance of the vendors' offer to compensate for deficiencies and effectively "waived" the applicable condition precedent, hence there was nothing more that needed to be done. The vendors did not have to sign their acceptance of that to which they had already agreed. The absence of their signatures did not result in a nudum pactum, a thing writ in water.