The CAP 804 document hasn't been maintained for nearly a decade. Current consolidated versions of the retained EU Aircrew Regulation and associated implementing rules, along with the delegated act that deals with foreign conversions and validations, are available from UK CAA
here. The AMC/GM was adopted by UK CAA in
ORS 9. Some derogations or exemptions from Part-FCL will still be found in
ORS 4.
The version of FCL.035 point (b)(5) you cited has been amended to refer instead to FCL.720.A point (a)(2)(ii)(A). You can disregard FCL.035 point (b)(5) which refers to crediting across aircraft categories, eg aeroplane to helicopter. The requirements in FCL.720.A point (a) apply to single-pilot high-performance non-complex aeroplanes regardless of any provision in FCL.035.
As rudestuff points out, neither FCL.025 nor any other rule imposes a validity period on the ATPL exams for the purpose of FCL.720.A point (b), viz the requirement to have passed the ATPL(A) exams before starting training for the initial
unrestricted MPA TR.