<Is the procedure designed with the absolute worse case scenario in mind. ie Max TOW, ISA +35, low QNH etc and the resulting gradient? >
Generally speaking, the TOW allowed WILL assume you will fly the emergency turn for E/Out terrain clearance, and will be calculated for the ambient temperature and QNH. Thus MTOW will be lower if the temperature is higher or QNH lower. Most ETs expect you to follow the initial part of the SID (presumably you WOULD adjust for drift there?) or specify a heading, track or turn. All such will allow for a reasonably-to-be expected adverse crosswind. Unless a specific E/Out G/A procedure (eg Glasgow R05) is specified, it is 'safe' to assume you will be ok up to the max allowable landing weight for that runway.
All the above assumes, of course, that your company has got it right!
I suspect you need to look at PansOps in Europe for guidance. I expect one of the 'fundis' on this forum will come up with a link for you soon.