I can answer to your first two questions. A -5 event may reduce some Homeworld to Deadly level, or, rather:
0.0-0.2: Deadly
0.3-0.4: Harsh
0.5-0.9: Unpleasant
1.0-1.2: Mild
1.3-1.4: Good
1.5: Optimal
According to this chart, all Harsh planets will be Deadly (with a value of 0.0) while half the Unpleasant planets will become Deadly (with a value between 0.0/0.1 and 0.2) The other ones will simply see their conditions drop. Granted, a -5 or a -20 event on an Upleasant planet (with a value of 0.5) will be equally as destructive.
However, you have mentioned the possibility to add +8% and +16% conditions improvement facilities. If so, you would need only eight years with the latest facility to go from Deadly (0.0/0.1) to Harsh (0.3) compared to 39 years with the regular +3% facility of the vanilla game. And of course, you could still add a few additional facilities to speed up the process.
As for these events, well... They are only appalling when they target your Homeworld. If that is a matter to you, perhaps you could add a condition improvement ability on the regular Cultural Centers but just for the Homeworld? (Unlike the Religious CC, whose ability is working on the whole system) That is, only if you fear this might be an issue. But as there will also be Heroes reducing strongly the odds of events, you may think of leaving the -10 event. (I forgot to mention these Heroes when speaking of these events before) Your choice there.
Good events are another matter, as far as I know, there are more bad events than good ones available, is that right JLS? It would make adding such events quite hard. I will give it a thought nevertheless.