@Andar - I actually understand where the OP is coming from. I don't see why there couldn't be a preview mode that only shows events that aren't switch driven. Primarily, I'd like an in-game preview for asethetic purposes, to see if my map and the mapping event layers, is the way I want them to be (since they tend to look different in game than on the editor) without having to first save.
@However, to the OP - an event preview (even if it were possible) would in no way remove the need for playtesting, and playtesting would show you whether the event works or not anyways.
The way I've been handling my current game is:
1. Remove all battles (random, touch encounters, evented battles, boss fights), and make the purchase price of every item and equipment 0. These will get added in later and tested separately. I still include the events for certain fights (like the boss fights) and the scenes within, but I remove the actual battle).
2. After I've mapped and evented a segment of the game, I load up my current save file and test the new area out. By segment I mean every few maps, a village, a dungeon, etc. What I'm testing is usually the cut scenes within, the various events that you can interact with (NPCs, shops, fetch quests, chests), the switches, variables, and so forth. Not having to worry about battles makes this much easier to do. Sometimes I will have to make multiple save files to test different branches of the game, especially when you have multiple events that can trigger the same scene.
3. Once I have play tested through the whole game (minus battles) and have verified all the non-battle content and am satisfied with how the game plays out, it's time to work on battles and balancing. Balancing includes price points for your items and equipment, amount of drops (gold, items, EXP) from each enemy, amount of XP necessary to level up, and how many enemies is necessary for each area (without overdoing it) so that you can be prepared for the boss fights that are sure to come. At this point, you'll need to also add fights to the events you previously had but left out the actual battle. This I find can take a long time, but it's a lot easier to do when you don't have to divide your attention with all the non-battle content (mapping, eventing, questing, etc).
4. At minimum, plan to fully test your game from start to end at least three times. The first time is the stop and go as you build your world and add your non-battle events. The second time is the stop and go as you build your battles and balance everything out. And then one last time when everything is done. Then, get yourself one if not two beta testers (more is cetainly merrier) to play the game as well and note any difficulties or problems they are having.
5. Finally, even with perfect planning, don't believe that three is the max times you'll need to go through your game; it's merely the minimum that I believe will be required for most people, especially if they don't have the experience behind game design. Whenever you have to drastically change the balance of the game play, you'll probably find yourself having to replay the game in whole or in parts just to make sure it works. In my own 8+ hour game, I wound up playing it through from beginning to end 5 or 6 times (do to contantly updating and changing some things) and I still found things I missed.
6. Bonus, I typically do most of my planning in multiple spreadsheets. I write down my switches and variables and what they do, and what number they are at, so I can find them quite easily. I also have to write down which events need to be updated down the road (with battles, etc), and I write a fairly thorough to-do list as I come across areas of game design that I'll have hold off until later.