No, only LED lamps that are suitable for it.
There are LED lamps that dim with phase cut and there are those with phase cut. Is the minimum wattage of the dimmer also important to allow the LED lamp to dim properly? An LED dimmer of at least 1w is best. Most old dimmers work on eg 20w. This means that you need at least 4 lamps of 5w to dim the LED properly without flickering.
No, some dimmable LED lamps are suitable for dimming.
Yes this is perfectly possible. A dimmable LED does not need a dimmer to work. For example, a non-dimmable LED will not work well with a dimmer.
This is usually due to a loss on your switch. You can solve this with eg an anti-flicker module. Or a toggle switch.
There are several factors that can cause an LED lamp to fail quickly.
+ Heat: An LED quickly breaks down if it cannot cool properly.
+ Grid pollution: One of the biggest causes of rapid defects is mains pollution by eg TL ballasts, energy-saving lamps, refrigerators, etc.