Same tactics to enforce DRM as it is on consoles...

Sell a trilogy of games, but the third one is a enforced download.

The Alien Isolation deluxe edition on PS4 is the same, digital extras -> account lockin with download or DLC on disc.

No thanks.

It does start small, I know that people ignore the nothing burger but that is how everything starts. The more leeway a company has the bigger steps they are encouraged to take in the wrong direction. People aren't perfect, so if one points out small issues and stops support it encourages the company to be "extreme" in their promises by keeping everything DRM free, but that ensures the company like GOG stays DRM free.

GOG Galaxy is not a DRM. It is a platform. Look up the definition of a DRM, it relates to management of content you buy. You didn't buy additional, free and optional rewards. Also, just remember you don't actually own the game or any of its content you buy. You own a licence to use it, the conditions of which can be altered or revoked at any time. It's all in those EULAs people skip.

If additional content (such as a mere T-shirt) usable only in singleplayer portion of game Z is OBTAINABLE only by means that explicitly require global network connection and online license verification then that IS a DRM by all means.

Incorrect. DRM being or not being a DRM has NOTHING to do with how much money you spend on something (for example no money).
Digital Rights Management applies to ANY content you obtain in any way.

How about you read GOG TOS for a change?

I don't believe, the game is fully ownloaable without the need of the GOG Galaxy... This DLC is exclusive for the ones who wants to use the Galaxy system, you don't have to use it to play the freaking game... Thus, the game is and will always be 100% DRM-free... Now shush an stop arguing baout exclusive items...
The game is DRM-free, they asks GOG Galaxy for this content only, you can still download the full game and play totally offline, but people are arguing cuz they want something exclusive for the GOG Galaxy platform, even if they dont want to use such platform...

This is just DUMB in another whole level...
Said DLC is exclusive for the ones who wants to play the game using GOG Galaxy... You still can download the full game from the site and play as is...

CD Projekt is turning into a garbage corporation

That is, unfortunately, true. They have turned their back on everything that made them stand out in a positive way.

I did, but I am no legal expert. Therefore I am not sure what I read.

User Agreement

Some sort of draft has a bit harsher wording than the current one. But in the current UA there's this:

11.1 Please follow these rules regarding the GOG services and GOG content. Please read these rules carefully since failure to follow them (particularly those in relation to cheating) will be considered a material breach of this Agreement, which could lead to suspension or cancellation of your access to GOG services. Here are the rules:

(g) Don't do or say anything which is or may be considered racist, xenophobic, sexist, defamatory or otherwise offensive or illegal. Be nice to each other please!

So your GOG account could, technically, be suspended or cancelled if you make an offensive joke. But only if they decide it is a serious breach:

17.2. Our right to terminate the Agreement. If you materially breach this Agreement, we reserve the right to suspend or cancel your access to GOG services and GOG content. By material breach of the Agreement we mean a serious breach which could cause significant harm to GOG, GOG users, as well as, in particular breach of the provisions of section 11 above. If we suspend or cancel your access to GOG services or GOG content we'll take reasonable steps to contact you to explain why we've done this and what (if anything) you can do as a result.

What is a "serious breach"? That is not defined. And in this case you would not be able to download the games for yourself if they decide to immediately cancel your account, since the 60 day grace period is for when GOG stops services:

17.3 It seems very unlikely, but if we have to stop providing access to GOG services and GOG content permanently (not because of any breach by you), we will try to give you at least sixty (60) days advance notice by posting a note on www.GOG.COM and sending an email to every registered users – during that time you should be able to download any GOG content you purchased.

So as I read this, technically, if I were to say "ppl who have pirate frogs as avatars r dumb", and there is no definite guideline what means "a serious breach", GOG could interpret this as a serious breach and cancel my account and with this I would lose the access to download the games I have purchased, leaving me only what I happen to have installed at the moment. I'm sure that this will never happen, but as I understand this it could technically happen.

I don't need reassurances that this will never happen -- I know this already. But I am interested in the technical side. Could this technically give GOG the right to cancel my account for anything they decide is offensive with no means to get the games I have purchased?

And a little side note:

11.1
(h) Don’t share, ‘buy’, ‘sell’, transfer, gift, lend, steal, misappropriate or misuse GOG accounts. GOG keys/codes can only be gifted or transferred or used in the ways permitted by GOG. If you have any questions or problems, contact customer support.

I think in 2018 EU got the first proper digital inheritance directives so in case of death I should be able to transfer my account to the next of kin. But I am not sure about this either.

No big deal. Unless you have so little of a life to care about such things.

I will put it this way:
As long as their TOS is within legal limits they can do anything - that is - their TOS needs to be within legal limits, about which I am not atm sure.
I can give you an example:
HumbleBundle does (or did, I wasn't following the case in few months) have TOS that is literally illegal in EU.
They "reserve themselves right to terminate your account and restrict your access to already purchased content" for basically "any reason".
And they did, as proven by quite some of their customers, block access to already purchased unactivated keys (sometimes even invalidating already activated ones) aside from banning corresponding accounts just because customer in question resold/gaveaway/whatever their legally obtained keys.
Here note:
1.YES. It does apply to GIVEAWAYS too.
2.It apparently also applies to "I activated this on my ACTUAL own non-primary Steam account".

It DOESN'T matter if "key reselling" is morally right or not.
What matters is that in EU law is such that once you pay for and get delivered legally obtained key at that point the original seller has NO jurisdiction WHATSOEVER anymore in regards to what you can do with the key.
Key reselling is 100% legal in EU.
Therefore HumbleBundle's actions are fraudulent and literally illegal.
It's quite bizzare that they have had been getting away with it for quite an extended period of time (they may possibly still be, I don't know since like I said, I didn't follow the case for the last few months).

So, back to your question, "can GOG do what they want"?
Probably yes because even if their TOS would be illegal they could still potentially get away with it if not "influencial enough" people would notice that.
And should you take legal route, they could likely hire lawyers to blast you to oblivion just because they have more MONEY than you (theoretically speaking).

I have no clue about this. And frankly I don't follow most EU regulations since major chunk of them is utter BS and I don't like watching "questionably competent" people at work.