Regarding Q3. Whopity doesn't have it quite right (very rare I know).
If you originally held a UK single-pilot IR (ME or SE), even if it was years ago, you can renew it via cross-crediting (the rules changed in 2019 to allow renewal as well as revalidation). You need to fly 3 instrument approaches in an SE aeroplane, including one RNP approach; as you don't currently have a SP IR, these should be flown with a suitably qualified FI or IRI. You can then get a CRE/IRR or IRE to sign you off.
If you never held a UK single-pilot IR (if you were ex-military or converted from an FAA ATPL etc) then you will have to do training as required and then an initial IR skill test with an IRE.