[general_re]

Both of your syllogisms are sterling examples of the logical fallacy of affirming the consequent, and are, sadly, invalid...

[Dimensio]

Actually, by prefacing the premises with "only Y, if Y exists, can do X", then it does logically follow that Y does exist if X is done. It isn't affirmation of the consequent because it is stated beforehand that the consequences will occur only under the condition that the premise is true. It's a difference between "if X then Y" and "if and only if X then Y".

[The Man]

Please elaborate. And how is the particular fallacy you mentioned not applicable to any syllogism? Is it not rather an attack on the truth or validity of one or more of the premises?