Freezing the fruits for a few days helps extracting the juice [H10, F1]
by breaking cell walls [R7] more efficiently than a food processor could do.
Do not heat fruits, to sanitize them as is done with honey, because the pectin of the fruits may set and create a haze.
When to add them?
If they are added at the beginning, they will provide nutrients (decreasing the need for chemicals) and they allow a faster clarification.
The drawback is that a lot of taste will be gone with the wind of CO2. If fruits are added after the end of fermentation, the batch can be split into several batches, some with and some without fruits.
Another hypothesis is that fruits added at the beginning of the fermentation are merely fermented, so the result is not a mead-fruit juice mixture but a mead-fruit wine mixture. As we know wine does not taste like grape or mead like honey. Therefore, if the fruit is added when the fermentation got soft, it will not or not much be fermented and the fruit flavor will be preserved.
Some fruits contain tannins as those found in red wine. This will add some bitterness to the mead but they can be used to add complexity. They are found in the skin and seeds (cores should be discarded for some of them can be toxic). So adding the whole fruit or only the juice will modify the flavor of the mead [MLD 907].