You might struggle - take a look at the suggestions on https://www.rhs.org.uk/advice/profile?PID=430. The most likely on the list is vinca recent posts on GoY suggest that you might regret planting vinca - if you succeed in getting it to establish itself it could get carried away!

Depends how close you want to plant - you'd need to leave a good couple of feet from the base of the trees, preferably 3 feet or more, so that anything you plant has a chance to grow and doesn't get deprived of water and nutrients because of the roots of the trees. Even so, you'd need to water more. How much space is available there and is it in deep shade from the trees?

Having watched one of the gardening programmes recently they were planting ferns under a large conifer and saying they would be quite happy in dry shade. I always thought that ferns preferred damp shade, but no doubt they know what they are talking about.

Most ferns do prefer damp shade, but there are 2 or 3 that don't mind dryness, Honeysuckle. If only I could remember the names... well, without having to look them up, that is!

I have planted Erodiums, Welsh Poppies and bulbs directly underneath mine and they all do well, I was fedup of the bare bit of ground, in the past I've had Marigolds and Petunias through the summer months, as you can imagine they are smothered in conifer needles and very shady but it doesn't seem to stop them, just make sure you give them plenty of water to get them started as the conifers are very greedy as you probably already know, I was told nothing would work but with determination and also by adding a load of my homemade compost into the planting space each year, I found that is untrue and learnt by trial and error, give it a go you won't lose anything...Forgot to say some of the Erodiums are very good at spreading and can become a thug..