What is the difference between strategy and technique?

Simply put, technique requires a particular skill to be effected. Not all techniques have a specific outcome in mind, instead being intended to produce effects.

Among the qualities that make Leonardo's work unique are his innovative techniques for laying on the paint; his detailed knowledge of anatomy, light, botany and geology; his interest in physiognomy and the way humans register emotion in expression and gesture; his innovative use of the human form in figurative composition; and his use of subtle gradation of tone.

Strategy requires adherence to a plan that is intended to lead to a particular outcome. Not all strategies require any particular skill. For example, Tic-Tac-Toe.

His well-planned strategies allowed him to conquer several Italian cities that were allied to Rome.

We do not speak of the amazing painting strategy of da Vinci, or the military technique of Hannibal.

As for your examples:

1. Kelsey had a good technique for watercolour drawings. Unless this is talking about something other than the skills used to produce a good watercolor, this is about her technique.

2. A cancer support charity raised £35,000 for charity due to its strategy of running charity races in scenic locations. Unless you mean the specific way in which the races are run by the runners come into play, this is a strategy to get people to turn out and give money.

3. A singer relatively unknown to the music industry was asked for an interview, so given that she creates media and appears in the media, she was very careful to have a good strategy/technique for how she conducted herself in the interview, what she said and how she said it. This one could be talking about either strategy or technique. It's strategy if the focus is on the plan to avoid saying things that won't paint her in the correct light. It's technique if there is a deftness to how she can verbally negotiate the pitfalls of speaking with a member of the press, who often have techniques to get people to say compromising things, and often employ a strategy of attempting to get people to say compromising things to get a noteworthy interview. Either one might be applicable here, but the sentence would mean two different things.