Also, the petals of those buds that have fallen on the soil mix, should I remove them from the pot or is it okay to leave them? Thanks

Many different insects visit flowers. Not always a bad thing. We need to see the critters, please.

Hi! I couldn't take a good picture as they are really small so i took one from the web which is a close match. Picture quality is still not good though. These beetles resides inside the flower buds and not on the leaves or the trunk of my tree. Thanks

These are pollen beetles. There are many different species, but generally they do no significant damage to potted citrus.

One of the difficulties of growing citrus in UK & Ireland is that flowering can be induced by a warm spell in mid-summer rather than early spring. Flowers and tiny fruit then don't develop further during winter and most drop off.

Fallen petals don't cause a problem unless they get soggy. This can happen if pots are outside in wet weather, and the petals start to form a slimy mess which provides good cover for slugs and small snails. Best to remove them before they get to this state.

thanks citrange! i think they may be the cause why those flower petals have dropped off significantly because my other calamondin tree which has budded first didn't dropped of its petals, they still clinged to the bud until they wilted.
i am really dreading winter time, i don't know what to do. should i keep my trees inside the house without getting sunlight or should i leave them inside a greenhouse?

If you can provide frost protection in the greenhouse, that is far better than dim conditions in a centrally heated house.