Is not incorrect. Is that I haven't explained myself enough. I don't want to write a large post.
1) A lot of people share their trade node. Yes this is right. But as England, I don't share my trade node. A screenshot describes it perfectly.
If you can see, Holland is the next one who has about 1/6 of my trade power. Basically I dominate 85-90% of the trade node and if you want you can declare a trade war and hurt them badly for 15 years. This is possible because mercantilism is so high (about 85% at the moment of the screen) and English ideas which gives embargo efficiency. This is way more complicated with Portugal because it's very hard to increase mercantilism. And Castille in the long term have more provinces to develop so they can hurt your income a lot. This can be solved of course, collecting outside of your main trade node but this is only useful if you absolutely dominate that node.
2) You have to waste a idea group where only one idea is worth it. So for me is not an option. It appears that devs have no idea how encouraging players to take espionage ideas, but in my case, raising liberty desire from mercantilism isn't going to work.
As you can see with maximum mercantilism + maritime ideas decision the embargo efficiency is (30% traditions, +50% mercantilism, + 25% maritime decision). So this is why I said in this case that GB is a exception. Literally I don't share my trade node with anyone. Even Holland with trade ideas can't earn a decent trade income. The problem with the colonies appear here:
At 100% mercantilism liberty desire will go up to +30%. Couple this with administrative efficiency and LD will go on rampage. My colonies are very small. I was focusing on India mostly and only taking ten provinces to increase naval force limit and merchants. As you can see British West Indies have only 5-6 provinces and 14% liberty desire. And -10% liberty desire is not going to solve this. You may say you don't need 100% mercantilism and that's right. But on a trade game your mercantilism is your prussian discipline so if colonies rebel at some point show them who is the boss
If you see relative power to GB is nonexistent. You know what will happen to Portugal when you have a fat CN like Mexico? That one of your rivals is going to support their independence. I mean if someone like one nation more than another is legit but don't trust the game advice. Portugal is not an easy nation to learn trade & colonizing because of their horrible starting position & development.
1) That's because it's holland and not the netherlands. When netherlands forms it usually dominates the english channel and you need to spam light ships to gain back trade power. Portugal starts in a different position since it shares from the beginning. It's more the difference of starts, but portugal does have the ability to conquer quenta. I'm also not sure what year it is in that screenshot, but i got 100% merc (1.19) by late 1500s/early 1600s. Catholocism is really good to get merch points. It could've been faster if i was conquering and converting lands but I mostly used (+3) merc events, papal influence, and spare spare dip point when 95% merc.
2) What's the wasted idea group? Diplomatic and espionage are all very useful idea groups for colonists, even in MP; Espionage more sore than the rest. maybe influence/offensive can be considered wasted but how is diplomatic? It's a very good idea to gain PUs and Protectorates with.
The same issue can be done with Sevilla. Ships + Merc + embargo efficiency (which is raised by espionage). I get your point, but the thing is, england, yes, starts in a
fantastic position, while castile, portugal and even france start off in a much worse one (at least trade wise). But, you can always bring these other powers up to exceptional trade levels. For example, I've played a french campaign where I had 50% trade power in english channel, Champagne
and genoa. I was collecting trade in all 3 making obscene money with only 30% merc and poor embargo eff.

France trade, granted it was to play around with 1.19 beta so there was no real goal.

Also, granted, france was obscenely big (Gb is under a force PU like Provence and others were lol).
I've played GB plenty of times (the most out of any nation), so trust me i understand where you're coming from. I'm not saying you're wrong either, however you need to factor in 'starting' situation into your assumptions. GB has an exceptionally strong start trade, military and economy wise. So using GB as a use case is not entirely the best method. But let me show you a few screenshots to present something to you.
First of all, i need to apologize because this was an iron man save, so I don't have access to portugal directly (I was going for a few achievements including
turn the table which I realise I failed at since it can't be achieved by releasing a colony, hence the reason i quit the save).
Notice the size of Brazil (Just released) and the size of Portugal. Portugal is only as big as the screenshot shows, only extending to ivory coast since i wasn't playing a serious portugal campaign, only colonising brazil to attempt an achievement or two.
Notice the Force limit of brazil and portugal (Add about 10 to portugal and remove about 6 from brazil, since brazil was ex colony).
Notice the Dev of Brazil (409) and portugal (363).
Notice portuguese ideas, none of which are meant to affect lib desire, but mostly colonial speed (last one was picked by AI).
Notice that Portugal has 60% trade power in sevilla. Notice there's also barely any ships protecting trade (less than castile and far less than your england screenshot). Portugal's Trade power is also noticeably lower than your GB's, yet it has 60% trade power.
Finally, Notice that portugal has 200% trade power Eff from mercantilism, Meaning it has 100% Merc.
With all this data, Brazil was never above 50% Lib desire. In fact, I'm not even sure if they reached >50% lib desire even when I raised tarrifs to about 140% (100% + tariff bonuses), just to test how much money i could make off of that huge brazil just before release (it was about 60 ducats in tariffs, but I forget the exact figure).
I'm showing this, not to tell you that you're wrong, but to show that your data is incomplete and that scenario you've portrayed with england can quite easily be replicated with portugal. In fact, The data of portugal here could be far better if i actually planned a serious portugal campaign from the start instead of simply trying to get a few achievements (
a protected market and
not so sad a state... among them).