Your question is slightly unclear, but I'm assuming the following:
- You're asking about the US/FAA Part 61 requirements
- The candidate is planning to take the CSEL checkride
- The candidate holds a private ASEL certificate, without an instrument rating
- The candidate has received some MEL training (you didn't say how much), but isn't yet rated for MEL
In that case, the answers are:
- 61.129(a)(3)(ii) requires 10hrs of instruction in a complex or turbine aircraft, but it doesn't say how many engines the aircraft has to have or what the training has to include. So using time from general MEL training is fine.
- The FAA has said that 'double-dipping' time from private to commercial is not allowed, therefore flight time as a solo student cannot count towards the commercial certificate (Theriault (2010) interpretation).
- If the pilot was receiving instruction in an aircraft that they weren't rated for then there's no way for them to log PIC time because none of the scenarios in 61.51(e) for logging PIC time apply. As you said, logging PIC as sole manipulator of the controls also requires you to be rated in the aircraft.
(This question is related.)