They keep together in the centre to keep warm and scatter when disturbed to startle predators (or arachnophobes in this case! ) Just think of all of the thousands of greenfly they will catch and eat in their short lives which would otherwise be damaging your plants!
Could it be getting too hot? What is the ventilation like?
The best and cheapest way is to buy a bare-root tree which are available from November to about March. However, you can buy potted trees which can be planted at any time but are expensive (I would expect them to be £100 upwards) and you want one supplied in at least a 40 litre pot. Here's an example website where such trees are available:
You need to rub them off (suggest wearing gloves) or use something like a nail brush to brush them off. Pesticides generally don't work on the adults so spraying isn't an option.
They are nematodes of some kind and are generally harmless or even beneficial. All soil contains nematodes but most are very small so go unnoticed.
Sow them when the seed pods are opening on the ones you are already growing. This gives them time to establish before they go dormant over winter. Spring sowing works but you usually have to wait a year before they flower.
1) looks like a carrot - maybe from birdseed. It could be one of a few other things but is definitely not a weed so would leave it.
2) is a poppy.
3) don't know but is another 'not a weed'!
I buy mine from Wickes. As long as it is 'sharp' or 'grit' sand and not 'builders' sand you should be OK. Sharp sand is gritty whereas builders sand has round grains. No issues with lime or salt with the stuff I buy.
It's a variegated weigela, possibly 'Florida Variegata'.
Here are some that aren't fussy and grow quickly: African daisy (Osteospurmum), Alyssum, Calendula, Candytuft, Coreopsis, Gypsophilia, Impatiens, Marigold, Mimulus, Nasturtium, Phlox.