I've written for pretty much every single type of wedding scenario and family make up you can imagine. Every time I speak to a client who warns me their particular situation is going to be uniquely challenging, I brace myself for a story I've heard several hundred times. Brothers marrying the other brother's ex wife, partners stolen from friends, family feuds that mean nobody is on speaking terms, and even a dog making a speech...are all part and parcel of what I cover. I always like to say that the rarest wedding scenario I write for, is two couples in their thirties with two intact sets of parents. It doesn't happen.
After several decades of people waiting until their mid thirties to marry, and a healthy appetite for second marriages, it does mean the likelihood of one or more of the parents in a groom speech not being around, is significantly increased. I cannot remember the last time I wrote for a wedding where the absent friends toast didn't feature. Anecdotally, they may change over the next 10 years, as many more people are getting married in their twenties, but for now it's a staple of the groom speech. Sometimes it makes sense to pass this on to the father of the bride, particularly if it's the mother of the bride who isn't there, however, I would avoid giving it to the best man as that would jar with his comedy output.
The best way to handle talking about people who have died, is t keep their inclusion in the speech as positive, uplifting and celebratory as possible. Avoid going down a mini eulogy route at all costs, because is neither the time nor the place. You've also got to keep it really efficient. I know that's going to be hard if it's your parents you're talking about, but sentiment is always much more powerful if it's said once in a really great way, rather than 3 times same the saying the thing. Despite the temptation, you have to avoid going into detailed stories or memories or stories, just say what they meant to you and your now wife in a really creative, warm way. Humour, also plays a big part - if you can think of something funny you can bring in about their character to lighten the load, then their part in the speech will be all the better for it.