As for the offensive, if found to be true then we should be applauding them. Nobody should sit idly by while their people are senselessly murdered by Putin, if you get an upper hand, no matter where or how slight, you take it.
No.
Ukraine will have a hard case to make if seeking western support for an offensive military stance, - especially if they seek western military support, weapons, equipment to do so - and few western governments could justify arming them, or supplying them with military equipment, were they to embark on an offensive of any sort against Russia.
This is something that would be most unwise, diplomatically, militarily, and politically.
In fact, if this was proven, western governments would be quite within their rights to cease supplying Ukraine with arms, - to use them to defend yourself is one thing, to use them to attack Russia is quite another - and, moreover, Russia would be quite within its rights to call out the west for supporting (and arming) a government which had been proven to have attacked Russia.
No western government would want - or wish - for this. And neither would their electorats. An armed conflict with Russia would not be in anyone's interest.
Ukraine's strength - in moral terms - is that they are the victim of a groundless, illegal, unprovoked and unjustified attack, and that they are defending themselves, and seeking to preserve their state and their independence - against an especially appalling and odious opponent.
Meanwhile, for now, - and bear in mind that the Ukrainians have denied it; moreover, these days, given the respective reputations of the parties to this conflict for telling the truth, the Russian narrative may not be believed, whereas the Ukrainians can expect to be taken more seriously, as what they have said to date is a lot more credible.
In any case, I suspect that this is a classic "false flag" operation on the part of Russia; the war is going badly for them, and they are grasping at straws to seek to galvanise public support (domestically).
Thus, until otherwise proven, I will assume that the Ukrainians were not responsible for this action.