Any fruit, which is fresh is always better. However, due to seasonality of the Amla fruit, it is not available throughout the year. Hence, for the sake of convenience, it can be frozen and used as and when required. I would say, it is not as good as fresh amla, but still good enough to use.

Read related: Amla remedy for excess body heat

Yes, all sour foods and herbs, including amalaki, are incompatible with milk, because Amla curdles it.

Solution? You need to give a gap of at least  30 minutes between intake Amla and milk. For example, if you are prescribed Amla capsule, along with Turmeric capsule with milk, then take Amla capsule before food, have food, and then take turmeric capsule with milk. This will ensure that the milk and amla are separated.  

In Astanga Hridaya, in vajikarana chapter, Vagbhata prescribes amla powder with milk.

In Ashtanga Hrudaya, Uttaratantra, Vajikarana Chapter (40th chapter) – 27th verse, there is a reference of usage of long pepper, amla, processed with amla juice, sugar candy and honey. – Here, milk is not used in processing the product, but is only used as Anupana. Moreover, by processing it with honey and sugar candy, the product would be sweet than sour. Hence, milk as after drink can be substantiated.

I read in Wikipedia that hidrolizable tannins can be hepatotoxic.

Tannins only in a very high dose, amounting to 20-30 amla fruits per day would be toxic. But no one takes that amount of amla in a single dose.

Amalaki powder gets absorbed and shows effect very quickly than tablets and capsules. More often capsules contain only selected extracts of the fruit rather than the whole herb.

Ayurveda recommends whole herb than extracts.

Yes. But it is always best to give a break once in a while, when using any Ayurvedic herb. If one is taking this fruit throughout the year, because it is a coolant, consider stopping it during winter.

No. Usually coolant herbs are styptic in nature. Amla is a coolant fruit. This is why, it is used in bleeding disorders such as nasal bleeding, menorrhagia. In children with repeated nasal bleeding, amla hair oil is advised for the same reason.