Early churches probably didn't sit. I'm basing this on current Orthodox praxis, and that the Orthodox church is very reluctant to change anything. The praxis is based on a NT verse - something about "standing before the Lord", but I don't recall the detail.

In mediaeval times, some churches were built with stone benches at the bases of the walls for the use of the old - hence the saying "the weakest go to the wall". These can still be seen. In Britain at least (I think also more widely), monasteries had individual seats for the choir monks (as opposed to the lay monks), facing across the church. These can be folded up to allow more room for standing during some parts of the service, but a common feature is a "misericord", a small ledge which is at bum height when the seat is folded and the monk is standing. The word means "pity" or "sympathy", and was supposed to give some support during long services.

I'm not sure when pews for the congregation came in, but they seem to have been present in some parish churches prior to the Reformation hitting England. My reason for saying this is that due to the increased important of preaching, some churches changed their seating layout to face a pulpit on what used to be the side of the church, so that the church was now wider than long, allowing the congregation to hear the preacher more easily. This was sometimes done imperfectly, so that some of the box pews face in the wrong direction still.

A box pew, btw, is a pew with a door, and tends to have higher partitions than is needed purely to provide back support. These would be rented by a family. These were later replaced by lower open pews to be used on a first-come-first-seated basis, although you can still find churches with a mix of box and open pews. Box pews are generally found in older Anglican churches, but were probably more important in non-conformist churches at first, since they were not supported by tithes and had to find other forms of income. In Scotland, the annual auction of pews could cause some friction (see JM Barrie's Auld Licht Idylls for a fictionalised account).

I started to do some research in this area, but my findings are still preliminary.

I don't think, as /u/ctesibius says, current Orthodox practice is necessarily a good guide to early church practice, but in this case it does seem correct. At least, in my main period (2-5th century church history), congregations would stand, and often the presiding minister would sit when preaching.

There's a document here from 1844 that gives some argument about pews and their origins. So at least in England the introduction of pews precedes the Reformation.

I think the argument that Protestantism accelerated the adoption of seating in church due to the more prominent place of the sermon is not necessarily proven. It sounds like a sound argument, because the Reformation did place greater emphasis on preaching, but I can't find any evidence that this fact itself lead to wider adoption of seating in churches.