First, when you are inviting, do it very discreetly if you are at the office. Even if one your co-workers finds out, just tell them the truth (unless it is because you just don't like them). If it is because you do not like them, I can not help you at all. Sorry if I did not help. :(
Yes it is etiquette to have a bridal shower for a second wedding.
It is proper etiquette to only invite people to the shower who will also be invited to the wedding. If the couple is having a private ceremony with only family, then only invite family to the shower. If they are eloping and not inviting anyone, it would be safest not to have a shower because those at the shower would be expecting a wedding invitation.
Ask the bride if it is ok, unless you want it to be a surprise. Then, ask the groom if it is ok.
The mother or sister of the young woman expecting a baby can have a baby shower if they like. Often it is a sister that can arrange a baby shower and your sister will probably have another baby shower at work or by one of her friends.
No. There is no rule of etiquette requiring you to invite a roommate to a baby shower. If it is a friend or relative, you will want to consider the impact that excluding this person would have on your relationship. If you are having a couples' or male & female shower, you need to invite this person if they are a significant other of a person that is invited.
Ideally, the bridal shower would be held 1-2 months before the wedding, but there is nothing wrong with it being held more in advance, or less, provided that the bride can attend.
Etiquette concerns the comfort, care, and well being of others, and others only. It is entirely selfless and absolutely destroyed by selfishness. So etiquette does not see this shower already given. Etiquette does not care what you've done, but what you are about to do. Etiquette says if you can give her a shower, do so. If not, find someone else to help or let her know gently. It may seem like a lot, but this is love, caring, kindness, generosity, and all those other virtues that without with, etiquette would not exist.
A mother can give her daughter a bridal shower (generally the guests would be relatives) but, it is usually the maid of honor that gives the bridal shower and perhaps coworkers at work.
Phone the person that is having the baby shower and let them know that you are sorry, but you cannot attend. Generally the host of the baby shower will accept that answer and seldom ask why. It is proper etiquette to let the person know that you won't be attending. A nice note to the mother to be be would be nice and apologize for missing the grand event.
The focus of a baby shower is on the mother-to-be and her baby. The father or baby-daddy or whatever is generally not present, married or not. Just keep the focus on the right things -- the positive aspects of parenthood and new life. The shower is not a proper time to create any bad feelings or offer any opinions on lifestyle.
Since the bride is never suppposed to be throwing the shower, it's up to the bridesmaids. It may be seen as a money-grubbing opportunity by the friends and relatives, especially if she didn't send everything back.
'In the shower' would be correct as the world 'shower' does not strictly define the shower head.