It is obvious that you need to replace the rubber draught strip, yes.
And yet the problem has persisted for at least 6 months and 3 seasons.
If you don’t replace the draught strip how will you know if it needs anything more?

I’d be thinking fibreglass may and resin over a wire mesh form. I’d still want either rigid foam fill or wood struts to strengthen it inside.

If you can make a central cut out or have one made, that would be secure, but you’ll also lose a bit of height that way so you’ll need taller blocks. The width of the blocks is important for stability.

Ok, 25cm is the typical highest bed leg raiser and they tend to rely on being compatible with a specific design of bed leg to attach securely.
For stability, if you don’t have the skills to construct a stable frame under the legs, I’d consider solid blocks of wood 30x30cm.
If you search for those you’ll find them on Etsy, etc. They’re not super cheap (£30 or so) but they’ll last and other sources like timber mills will be cheaper.

A piece of non slip mat under each one and between bed foot and block will keep the bed from gradually sliding over time.

If it’s stuck down in the U bend undo it as already described:
If it’s a big cap and it’s sitting just below the plug hole you’ll need to either: poke it up from below (after undoing the u bend) or poke a hole in it from above and hook it out.
If it’s wet it’ll be very hard to make anything stick to it but if you can get a wire coat hanger and undo the twisted ends you can heat the end with a lighter and poke it straight down through the plastic cap. Then twist it so the wiggly end goes in like a corkscrew and pull the cap out.

How. High. Do. You. Need. To. Raise. The. Bed?

Nice new threshold.
That pvc is an abomination though, waste from a downstairs toilet?

A slim non slip silicone case for the tablet would be simple.
Or a piece of the non slip mats you can use on a boat table to stop plates sliding off.

https://m.media-amazon.com/images/I/7185U%2BG%2BGKL._AC_SX300_SY300_QL70_ML2.jpg

This sounds like a scam.
How much insulation is in the attic floor?
A cold loft has floor insulation and lots of ventilation to keep the moisture rising from the house below moving outside. There should be a gap along the top of cavity walls and a draught blowing through the loft.
A warm loft is insulated under the rafters with air moving around the timbers above the insulation.
Insulation under the tiles with air blowing around the open loft space defeats the object.
One advantage though, would be to reduce heat transfer through the roof in summer.

I’d use either JB Weld or high temp oven door silicone in the hole.
If using JB Weld etc and you want to be able to remove the screw in future, use a toothpick to spread a layer inside the hole, lightly oil the screw and wind it in, compacting the JB weld around the top of the hole so it’s a good fit. Leave to set. The screw can be unscrewed leaving a new thread because the resin won’t stick to the oil.
Same with the silicone but the thread won’t be as strong.

Reading up on it there doesn’t seem to be a special trick, just push the spindle lock button and twist anti clockwise.
However, keyless chucks do sometimes bind and one trick is to tap the end of the drill bit sharply with a hammer. Obviously this might damage the end of the bit, so wear safety glasses.

Can you give the drill model number?