If I may offer an opinion based on my personal very rustic understanding of Quranic general principles I would say the following:
For 1 (i) Acting as a guest: It would seem to me that the objective of being a guest is to celebrate the event taking place, and perhaps also act as a witness to the event taking place. Acting as a guest may also somehow lend credence to the event if viewed from a social perspective.
As you have also noted, the Quran implores that we enjoin what is right and forbid what is wrong.
For these reasons I would agree with you that attending as a guest may not be what a good Muslim ought to do.
For 1 (ii) Acting as a service provider: Whilst it is true that a service provider may not know for certain to what end a service may be utilized for,
if it is known that a service is to be utilized for some evil or frowned-upon purpose then I think it is better to avoid/refuse to provide that service. Again the Quranic verse on to enjoin what is right and forbid what is wrong would seem to apply.
Consider a scenario where you are a manufacturer of weapons. If you know that the weapons you make are intended for use in the indiscriminate slaughter of women and children then would you still sell?
There may however be situations where there is uncertainty in understanding the ultimate ends of the client. For such situations perhaps it is better to pray to God and seek inspiration for the best way forward. Perhaps you may have a gut feeling for what to do. Remember that God provides all. Therefore what may be perceived as a lost business opportunity may not really be so as long as the heart and intentions are in the right place.
For 2:
Joseph Islam's recent article on Understanding Trials may be a useful guidance:
http://quransmessage.com/articles/understanding%20trials%20FM3.htmFor 2 (i)
Genetic disorders like GID and Homosexuality I think are not sins on their own. However the fostering of these inclinations in the mind perhaps and the acting upon these inclinations is perhaps sinful. As JI has explained, based on the Quran, we all have our trials. We are asked to deal with them as best we can and do not transgress the limits set by God. We must be honest with ourselves that we are doing the best we can. We can however take assurance that with "hardship comes ease" (94:5-6) and that "On no soul does God place a burden greater than it can bear" (2:286)".
For 2(ii) and 2(iii)
The Quran asks us to pursue a moderate path and go not into extremes (I forget which verse). I personally would not class surgery to alter gender and eyesight correction in the same category. The former is a massive change to alter the identity of an individual, the latter an oft utilized and mass produced solution to a well understood problem.
I would focus on the Quranic principles of patience and perseverance. Whilst the individual may struggle socially to manage gender identity or homosexual inclinations, some faith in God and an understanding of the true nature of the world (trials, on no soul is placed a burden too great and with hardship comes ease) is helpful in dealing with these issues I think.
Anyway, that's just my personal view. I hope it helps!