it shouldn't matter if it's fully open on a properly designed system, i would be tempted to open it fully up & only close it down slightly if you have an issue with the rads when both hot water & heating are working
open, although i'm guessing it's there to balance the system, as you know you didn't have to touch this, if you can remember how many turns you did to close this off then open it back up by the same amount
no it should be pumping towards the zone valve, you need to turn the pump body around so the arrow is facing down, plus if you have had a new boiler fitted then you should have had another zone valve fitted on the supply to the cylinder & had the single channel prog changed to a twin channel...
that vid doesn't tell you what the prob is, as they stopped & pulled back before getting to the blockage or damaged part, looking at the joints some of the drain has moved but still looks intact & doesn't look like it's dropped to me, they or someone else needs to find the cause of the problem...
the isolation valves you intend to fit, will leak & need replacing far soon than the element, so you trying to save yourself work will infact cause you more work & expense, just fit the rad valves & don't worry that you might have to change the element in the future
you can't use the same fibre washer, it needs a new one, that's providing you haven't already done as Hugh suggested
have you replaced the doughnut washer as well (thick rubber one between the cistern & pan) because this will need a new one as well