@KD, you're interjecting non-existent mechanics into the game and conflating actions with powers. What you're saying is simply not supported by the rules.
Actions do not have targets. They do not have to be planned start to finish. There are multiple examples in the rules which clearly demonstrate this to be true. Powers have targets. Even in the case of powers they may have more than one effect and the player may be able to make choices during different stages of power resolution.
Examples: A Move Action (Walk, Run, or Crawl) does not have to be planned out in its entirety. The character clearly moves from square to square and may decide to stop or continue moving as circumstances dictate. At no point are they obliged to "plot out" the entirety of the action ahead of time. This establishes that actions, contrary to what you've stated, are not immutable. The player can make decisions during the course of the action. At no point is it stated in the rules that this is specific to move actions either. Movement in general is very flexible and these kinds of decisions are most often made during move actions, but no rule states that this is the only time this is possible.
Standard actions can also involve decisions. When a character takes the Use A Power action and uses the Twin Strike power he gets to first choose a target, resolve his attack against it, then choose another target, and resolve the second attack. This is clearly established and there is a FAQ entry stating this outright. Additionally Immediate Reactions can come between the first attack and the second, which again underscores that the action has independent stages. This is clearly RAW.
Thus we have established that players are allowed to make decisions during actions and need not define all of those decisions beforehand.
Charge is an Action, it is not a power. Thus, at least potentially, it is possible for a player to make decisions during a charge and no rule in the game, including the charge rules, states that these decisions must be spelled out ahead of time. This is RAW.
Really it is that simple.
Furthermore nothing in the charge rules dictates at what point the validity of a charge will be determined. It is never stated that the player must map out the route of the charge and validate it against the conditions for a legal charge before charging. It is never stated that there is a single identifiable designated target for the charge. It is never stated that any decisions relating to the MBA power use which happens at the endpoint of the charge is being invoked at the start of the charge and thus no rule requires that its target be selected at that point.
In fact consistency with the rest of the rules, which allow decisions during actions, would tend to support the position that the various decisions required to execute a charge are, just like other actions and powers with multiple steps, made during execution of the charge. In other words the player can choose which squares his character will move through, where the charge will end, and when and against what target the MBA will be made as necessary during execution. This also indicates that the validity of the charge is evaluated on the basis of those decisions which have already been made and at any time during the charge at which it becomes a valid charge movement the MBA can be executed.
Now, it is never stated anywhere in the rules what happens if the character either chooses to make a decision which leads to an invalid charge or other circumstances cause it to become invalid what happens next. I think personally that the most logical procedure would be to simply declare the charge at that point to be governed by the rules for a normal movement action (Walking). No rules really say one way or another and the DM would be perfectly within his or her rights to do whatever they want at that point.
Thus I stand by my interpretation, which is making the least possible number of assumptions and extrapolations of the rules. Applying it to the OP's cases I would say if F causes M to stop moving then F has a decision to make. M can evaluate charge validity vs F and possibly issue the MBA against F. Otherwise the action is over and M can initiate some new action potentially depending on what types of actions it has remaining, etc. In the case where T's location changes again M may have various options. It may still be possible to execute a valid charge against T. It may be possible to continue the charge and execute it validly against some other target, T2. It may be that at this point M discovers that there is NO possibility of executing a valid charge anymore. This is the undefined case, but again it seems reasonable to allow T to finish out the movement portion of the action since that doesn't result in M doing anything it couldn't possibly have done anyway (by downgrading its standard to a move).
Show me the rule that says any type of action must be plotted out ahead of time anywhere in the rules. It doesn't exist.
