It strikes me that the school / rental operation is being a tad OTT here, but maybe erring slightly on the side of caution.
Specifically:-
- You aren't asking to fly with A.N.Other pax in the back seat, you are asking to fly with another PPL. In that position they should know enough to realise any risks and accept them, and also to handle any airsickness issues.
- Nav? Apart from flying a little bit faster, and a slightly poorer view out of the window, I fail to see any significant difference between navigating the PA38 that you are used to, and this PA28.
- There is absolutely nothing even faintly scarey or unpleasant about stalls, low speed, steep turns etc. in a PA28 - it is one of the most docile aeroplanes I've ever flown. I'd personally be quite happy practicing these with my Granny in the back (mind you, I have a very remarkable Granny!).
- If an FI has a problem doing a stall or 60° banked turn in a PA28 at MAUW, they have a serious confidence problem as an instructor. I'd find another instructor.
- Nobody can have too much time in the air, even if it's only sat in the back watching the bloke in the front being checked out.
Okay, yes, it's their aeroplane and they are setting the rules. Also, by any post-WW1 standard your 90 hours is low experience, but nonetheless I'm unconvinced that this degree of precaution is necessary or appropriate.
Plus, it's a heck of a lot easier putting a spare pilot in the back than lugging around and securing ballast bags, always assuming that the school have any.
So,
PPonting I personally think your own view and approach are entirely reasonable, probably more so than those of the school.
Of course, it may be that the school know you and don't think that you can fly for toffee - in which case they should just say so !
G
N.B. On a practical point, I've earned my living on and off over about 15 years as a Flight Test Engineer, which often involves sitting in the back/right of an aeroplane whilst a test pilot does their best to make me airsick, and a few have come incredibly close to succeeding. If I may offer a few hints here: (1) don't eat for a few hours before flying, (2) keep the cabin temperature down and fresh air flow up, (3) if unsure of yourself as a pax, there's no shame in taking a non-drowsy type seasickness pill a few hours before flying
[do check the instructions], (4) If in doubt do something active (such as taking notes) and keep your head out of the cockpit as far as you can, and (5) take a sealable sick-bag anyhow - better still, two!