Hey Ronan,
I wouldn't take the reactions to your first post to heart. From what I have read around here the guys are pretty helpful. This is your first post in what is largely a forum populated by contractors from the US and Canada. Although there are a few members from the UK, Europe and even New Zealand who regulary contribute.
Your terminology and use of metric and a pitch in degrees is not commonly used on the other side of the Atlantic never mind by the guy stuck out in the middle of the pacific

. Residential housing techniques and building practises are also different.
That said, the advice you have been given has been sound, you really need to get a fully qualified structural engineers report on this one. The nailing which you have discovered that is not up to standard may be the least of the problems.
For example: If your roof span is 7250 that gives you a run of 3625 and at a pitch of 45° therefore a mathmatical rafter length of 5126. You can google the uk rafter span tables showing maximum unsupported span of a rafter according to depth of rafter, pitch and basic loading conditions. The props as you call them are not a structurally designed member and offer no or little suport to the rafter.
All bad news for the home owner by the looks of it and this is just one potential issue.
The folks here are trades people who took years to learn their craft and sometimes they view new members with suspicision and rightly so in my opinion. If I posted in a Mechanical Engineers forum that I was looking some advice on designing some supporting steelwork for a mine shaft what reaction would I get?
I hope you read this and it is of some help to you. If you need any further advice you can always hire me. Yes I am openly pitching for business :laughing: