I have no problem answering these questions, but as part of my answer, I like to inquire of the person asking, where in the NT do we get the idea that only a certain class carry out these functions? Can any of us point to a single instance in the NT where any of these functions is designated as exclusive terrain of a chosen few? Is it a commandment or an ordinance that only trained, seminary educated, ordained ministers be the ones to baptize, serve the Lord's Supper, wed, or bury? There is nothing wrong with them doing so, but are we not ALL Royal Priesthood, a Holy Nation, a Chosen Race?
So, to the practical outworking of how these things are carried out in simple house church settings...
The short version is that we deal with each situation as it comes up. In other words we don't worry about these things until in the natural flow of things they need to be dealt with. When the need arises, those in the house church leadership will call on us to help them. This usually entails sitting down and helping them understand what God wants them to do in this situation. Sometimes they come right out and ask us to lead the ceremony. Often I will agree to do so this first occasion, but next time it is their responsibility. I view these opportunities to further train and orient the servant leaders by their watching me do it.
Baptism. We don't make a big deal over who does the baptizing. Any disciple can baptize. In fact disciples are commanded to do so in Matthew 28:18-20. Usually the way this works is that the house church leader will do so themselves with one or two assistants from the church. If for whatever reason they are not able, or do not wish to do so, they find somebody else to do the baptizing. It's not so much WHO does the baptizing, as in WHOSE NAME they are baptized.
The Lord's Supper. The Lord's Supper is a meal and regularly observed by the house churches. It is carried out in any number of different ways. One way is, again, to model how it is done. Many times when a group of new believers is ready for their first Lord's Supper, they will invite one of their mentors to preside. We gladly do so as a means of modeling a way of how it can be done. What is scary is that however we choose to lead during this time is often copied from there on out as "the way" to do the Lord's Supper! Over the years, though, I have seen a lot of creative and meaningful ways to celebrate this memorial.
Weddings. We have had many house church weddings over the past few years. Each has been special and meaningful to not only those getting married, but a blessing to the church as a whole. Sometimes I have been asked to perform the wedding, and have done so gladly. Usually though I will only perform the first wedding in a house church, but expect them to do any subsequent weddings. Sometimes the couple getting married will specifically ask their house church leader to do the ceremony. In these cases--and there have been several--the leader will come asking for help. We will sit down and step by step go over what needs to be done. We practice until they are fairly confident. It is important that the servant leaders be seen as empowered to carry out ALL the necessary tasks involved in church life. If we somehow leave the impression that only ordained pastors and missionaries can fill certain roles, we will harm the church's natural development. The last thing we want to do is create dependency upon the missionary.
Funerals. Again, we will go over with the house church leaders a basic outline of the kinds of things to say and do at a funeral. I just returned from one of these funerals not 30-minutes ago. The house church leader did a wonderful job. He asked me to say a few words, but the bulk of the time was led by him. I remember one house church leader being asked to preside over a wake. She had absolutely no experience or background to do so. In a panic she called several people to come to the rescue. None were available so she prayed to the Lord for guidance and went on to the wake. There, she was able to minister in the power of the Holy Spirit, and was a great blessing to the family. She related that it wasn't that hard. It was just a matter of allowing the Spirit of God freedom to minister through her. She related they sang a few songs, she shared a passage of Scripture and a few words of comfort, the family shared their memories of the loved one, prays were said, and then she visited with the family.
The list really extends to many other natural church life functions as well. Praying for the sick, dealing with demons, counseling, baby dedications, home visits, anniversaries, birthday parties, etc. NONE of these are the exclusive domain of professional clergy. ALL are matters which normally should be carried out by Spirit-filled disciples. It is not about us and how highly trained we are, but about HIM and what He wants to do in and through us.