Okay, separate topics.
How much can you prune off roots when repotting? As said above, there is no fixed answer. Much depends on the species of tree. The health of the tree prior to repotting, the timing of the repotting, and the effects of your local climate, and the diligence you can provide for after-care. This all comes from experience. If your trees are still alive 4 months after repotting you probably did not cut off too much. Did you pay attention to how much you removed at repotting? Take pictures if you have too many projects to remember specifics. Experience is hard earned. Most deciduous trees will tolerate quite drastic root pruning if they were healthy before the pruning and the timing relative the the tree's growth cycle was favorable. Conifers in general are less forgiving of excess root pruning. Aftercare is important. You must refrain from inflicting further stress until after the tree has recovered. Biggest cause of tree death is excessive pruning to frequently after a traumatic repotting.
Preparing trees for winter. Key is to keep the trees outdoors as late into the season as possible. The cooling off at night to progressively lower and lower temperatures causes the trees to make metabolic changes necessary to survive the winter. Trees will increase the sugar content of their sap and cells, by decreasing the amount of water in these tissues. This makes the sap somewhat like anti-freeze. Buds will "harden off" with protective bud scales forming a tight covering over next years buds. THere is quite a list of changes. Your winters are relatively mild, compared to mine. But it is important to not keep the tree indoors for more than a day or two for display at this time of year. Just a few nights in the warm indoors can undo a months worth of adaptations to the cold. Specific critical time in warm to undo cold tolerance is not well documented except for commercial fruit trees, so as a general precaution in autumn and winter, when you bring trees indoors for display or a photo opportunity, try to limit the length of time. Hours are certainly okay, multiple days can undo winter hardiness adaptations leading to freeze damage later in the winter.