Learn English – the most appropriate pronoun for humanity

The selection of these vs. those is the same as this vs. that. (These/those are the plurals of this/that respectively.) All four are demonstrative pronouns in English.

Typically, the differentiation between this and that is related to the proximity to the speaker. This proximity can be either physical distance or even temporal distance.

This is my house. (Standing on the front porch.)
That is my house. (Standing across the street.)
These are my shoes. (Holding them.)
Those are my shoes. (Pointing to them.)

The problem you are having in explaining the example above comes from the inherent ambiguity of an abstract notion. It is hard to explain the physical proximity to an idea or strategy, unless you are physically pointing to it on a chalkboard in front of you.

In other words: The example you gave above is correct both ways.

These two strategies help(ed?) Churchill get to this goal: expressing anger at the terrorists.
Those two strategies help(ed?) Churchill get to this goal: expressing anger at the terrorists.

You would have to decide from context and according to personal style which of these two pronouns better describes the idea in question. -- For example, because of the proximity in the explanation, I chose these for the preceding sentence. (Or if there are multiple strategies and you have picked out a selected few, I would choose those.)

With regard to his second use of the word those, this of course depends upon the need for demonstration. If Churchill was pointing the blame at a specific group of terrorists (out of a greater pool of known terrorists), then perhaps those would be appropriate. But, if it is just referring to the known set of terrorists previously discussed, then there is no demonstration needed, and the definite article the is perfectly sufficient.

I wouldn't recommend sending a valentine card in such a situation at all - Valentine cards carry connotations of sexual desire (or romantic intent, depending on how sensitive you are) intended to be sent anonymously to the recipient, despite the greetings cards companies trying to persuade people otherwise. It seems this would be inappropriate in these circumstances. Send a 'thinking of you' card if you must send something, or wait for her birthday or some other important anniversary.

Note that an expression such as 'you will always be my Valentine' specifically means 'you will always be my love' or even 'lover'.