Be careful here; intentionally taking an action that you know to be illegal, while seeking some advantage, will lead to some (majority of?) judges choosing to disqualify you. Your first example is clearly in that dangerous area, and I can't say I'd disagree with a DQ in that case - assuming, that is, I can figure out what in the world you were hoping to gain from it. ;)
The second example, if I read you correctly, asks if you can start to take an action and then - after gaining information - change your mind. This came up years ago, in a scenario where a player started to cast Wrath of God, using a
Chromatic Sphere during the announcement, seeing the drawn card, and then refusing to use a possible source of White mana to provide the WW needed to successfully cast the spell - thus, forcing a rewind.
If my memory serves (it happens sometimes!), a player was disqualified for doing that very thing. Not long after, the rules were changed - as evidenced by this 2008 ruling for Chromatic Sphere:
This is a mana ability, which means it can be activated as part of the process of casting a spell or activating another ability. If that happens you get the mana right away, but you don’t get to look at the drawn card until you have finished casting that spell or activating that ability.
In general, Mikaël, doing something you know you can't is a Bad Idea™. In your second example, generating sufficient mana during the steps of casting a spell, but refusing to finish because you didn't generate extra mana, is a very (dark) grey area, one I would not venture into, nor be very forgiving of.
d:^D
Note: I'll move this thread to the Comp REL forums, as I believe others will want to discuss it.