where do I find the official regulations about my last two questions?
Q3
If a pilot hasn't done 3 landings last 90days (no passengers rule) can I (as a FI) sit next to him?
FCL.060 Recent experience
(b) Aeroplanes, helicopters, powered-lift, airships and sailplanes. A pilot shall not operate an aircraft in commercial air transport or carrying passengers:
(1) as PIC or co-pilot unless he/she has carried out, in the preceding 90 days, at least 3 take-offs, approaches and landings in an aircraft of the same type or class or an FFS representing that type or class. The 3 take-offs and landings
If you are sitting next to them
as an Instructor then you will be PIC and they will be PUT.
Q2
If someone with a valid licence wants to do some training, can I log the time as instructing time?
Depends whether you are just along for the ride, as a passenger, or whether you are PIC and providing instruction
FCL.905.FI FI — Privileges and conditions
The privileges of an FI are to conduct flight instruction for the issue, revalidation or renewal of:
(a) a PPL, SPL, BPL and LAPL in the appropriate aircraft category;
(b) class and type ratings for single-pilot, single-engine aircraft, ……….
You cannot be using your privilege (a) as they already have a lifetime Licence; so you must be using your privilege (b) assuming that all and any Class Rating Instruction could be used towards Rating revalidation.
A new FI having to ask about their own privileges
EASA is too complicated - yes
Poorly taught FI course - probably