Ok, then this would be an interesting change. I would worry it wouldn't have a negative enough effect to balance the civic but it is elegant and focused exactly where we want the negative effect. I would like to experiment with it.
So how about:
Mercantilism -
No innate commercial bonus to international trade
Double yield from trade through your capital
Double production speed for custom house
Reduced destabilizing effect from colonies
Looks good so far I think. Might be a bit weak, but it seems preferable to Guilds when it becomes available. Production for Custom House is at Naval Dominance right now, but it doesn't need to be. Maybe the Plantation commerce idea to round it off?
Egalitarianism getting also the free specialist seems a bit to much... how about giving towns actually +2

? It might sound a bit much, but the +1 bonus always felt a tad too weak. However, you will not have towns and/or the civic until late in the game (especially if a late plague hits you!). Getting the civic with Democracy always felt right though. Mass Media might be an alternative, but it comes pretty late.
Maybe Democracy as a tech should be later in the tech tree. The 19th and 20th century are pretty light on philosophical technologies. The only reason it is where it is is to match the founding of the United States, which based on your definitions of representation and egalitarianism earlier in this thread was probably more representation than egalitarianism. The statue of liberty could be bumped to representation if you felt this pushed the wonder back too much. Maybe a prereq of railroad or assembly line rather than education would be appropriate.
Moving Democracy back sounds good, I agree that there are too few ideological techs later in the game. I think this civic topic really exposes how weak the tech tree is in some regards. Especially in the late game I want to make it more interesting, but right now we need to make do with what we have.
I think Constitution is an adequate representation of 18th-19th century political thought, and stuff like American democracy, which is adequately expressed by Republic + Representation, while things like European constitutional monarchies are Dynasticism + Representation. If Democracy comes later, it could be renamed Civil Liberties and have different prereqs. Not sure right now where it falls in context with other techs like Communism and Fascism. SoL at Constitution with maybe an extra tech requirement should be fine.
That doesn't completely answer the question which effects these civics should have. I think thematically Egalitarianism somewhat exists in the intersection between Representation and Public Welfare, so effects could be moved between those. Some additional ideas for currently unused effects:
- double Statesman slots (most appropriately Representation I think)
- reduced corporation unhappiness (suits Public Welfare in my opinion)
However things are assigned I feel that the free specialist is so strong that it should probably come with some sort of drawback in the same civic.
I understood also you think late game super production is a problem. Planned economy gives so many extra hammers compared to any other civic it should be addressed. Also historically planned economy was not very effective in terms of economy and commerce, so something to think about is as well.
Yes compared to its historical "success" planned economy is really strong. Combined with Industrialism, any city can get ridiculous production. How about -10%

, or completely forbid corporations?
I think historically Planned Economy has been pretty good at heavy industry, and it also includes stuff like modern China so I wouldn't be so generalizing with that statement.
I like that the production comes from specialists, but 1 is already the lowest possible value so there isn't much wriggling room for a nerf.
Perhaps the civic should have another drawback instead? Reduced corporation yield/spread? City maintenance?