1 - ID90/ID50/special refers to the price you pay and corresponding priority - they are all standby tickets.
Technically ID90 and ID50 stand for Industry Discount and are issued within the IATA agreements - 90 and 50 are the corresponding discounts, based on the full fare (usually the most expensive fare, such as full Y) for that cabin and sector.
90% and 50% are the discounts from that price.
These tickets are issued as a normal coupon by the airline under their own IATA code, so for EK that means a ticket starting 176.
Airlines can issue them for travel on other airlines as per IATA rules as normal - depending on the entitlement of the employee and whether the operating carrier accepts the ticket (i.e. will they get paid by the plating carrier).
For instance, you may find your wife's friend can herself travel on any airline EK has an agreement with using an ID90 ticket, but no relatives can.
There is also a corresponding AD90/50 for IATA agents.
A "special" is simply a travel authority issued by the carrier in question for travel on their own metal. A ticket may be issued in their own system in the normal way, but it isn't endorseable to any other airlines. The price is just whatever the carrier wants to charge.
2 - That depends on the entitlement of the employee. A CSV at EK does not have J/F privileges for staff travel
3 - No skywards credit - in fact, no skywards benefits whatsoever: no lounge, no additional baggage etc.
You are bound by the staff travel rules, especially the dress code rules, which your sponsor should tell you. Basically, don't look scruffy or cause problems for other staff.
Any complaints from EK staff about the behaviour of staff travel pax will be sent back to the sponsor's manager - punishment includes loss of staff travel privileges (in the case of a captain's wife who got drunk in F and was rude to crew, and aggressive pax arguing that because he was platinum he should be allowed into an outstation lounge) all the way to firing.
I would say that EK are looking to cut costs
4 - No