I have yet to discover all the altars and get to seriously trying to get the Velorien's Blessing, but I'd like to get Kerlin's +4 Persuasion and Haggle in the meantime (I think I discovered where the altar is, or heard a rumor at least).

Since (I think) the first god on the list, when doing it in proper order, has no problem with Kerlin, if I give them an offering later, I will just lose Kerlin's blessing and gain theirs (without getting a curse) ?

And will I still be able to get Velorien's blessing?

You need to do all the altars in the proper order to get the final blessing. (And once you do, you can't go back and get all the intermediate blessings again, FYI.)

Some gods don't like each other. So if you do the quest out of order, you may wind up getting cursed instead.

There's also a built-in limiting factor: there are only five total of one of the items you need to complete the quest; you'll need two of them (plus a third one for a different, unrelated quest) so you really only get one do-over.

Bottom line, this is one of two places (the other is a complicated late-game puzzle in a hidden city) where I absolutely recommend using a walkthrough. A few of the altars you just won't find, unless you either know exactly where to look, or obsessively criss-cross the land hoping to scope out their location. And even that won't tell you the right order to visit them.

I know as much, perhaps I have phrased the question wrong.

If I do that:
1) Go straight to Kerlin's altar and offer him a Mnura Coin (I have two, and recall some junk trader having a third, I'll hit them and see if they still have it) to get their Persuasion bonus for now.
2) Some time passes, in the meantime I access the areas with all the altars (I could probably just use coordinates from the Internet, but I prefer to do it at least semi legitimately)
3) I do the proper chain, from the first altar to the Velorien's one.

Since, as far as I can tell, the first altar's god has no problem with Kerlin, I shouldn't get cursed? And will the quest work properly, if I do it that way?

Edit: I tested it (after saving a game) and it seems to work the way I expected:

1) Made an offering to Kerlin, got a Persuasion and Haggle blessing.
2) Went to Falcon's Ache and made an offering there. Kerlin's blessing disappeared, got a new blessing and no curse.
3) Went to Bedokaan village, found a hearthstone and made an offering. The previous blessing remained and I got a new one on top of it.

Now I only need to find one more Mnura coin, since I only have two (the junk trader I mentioned didn't have it anymore), and see about spotting the hearthstones on the ground.

MILD SPOILER ALERT

.

.

.

.

.

.

.

.

.

.

.

Yes, it will work according to your expectations. But the first time you do it out of order, you will lose all previous benefits and have to start over. (And likely get cursed. Kerlin's opposite is Geshtianna, so you either didn't notice or care about the -1 penalty to Beauty your PC got.)

You can check whether any curses are currently active in your logbook. Click down to the Blessings & Curses tab, the one that looks like the yin-yang symbol.

Also, keep in mind that heartstones are the limiting factor: there are only five, you need two, and they're spread out all over the Bedokaan swamp. And if you're doing the Durin Stone quest you get from one of the Wheel Clan dwarves, you also need one of those heartstones to make the key from the schematic he gives you.

(Black diamonds are similarly limited, in that there are only two out-of-the-way places on the map where you can always find them; but you only need one to complete the quest properly anyway.)

I didn't miss it, just didn't go to Geshtianna, only straight to Kerlin to get his blessing (as I have only put enough character points in Persuasion, two I think, to leave the rest empty for the final blessing's effects, and that proved to be not enough, eventually), then to the first two altars on the in-order-list to test how it works - and going from Kerlin to elven god in Falcon's Ache doesn't give a curse.

As for the hearthstones, I found three of them in Bedokaan village, so I think I should be fine (and a black diamond in I think the disappeared dwarven clan's place, if memory serves. So I think I should be fine.

Thanks for the help, anyway. :)

I think you may have misunderstood. You didn't get the -1 curse to Beauty for going to Falcon's Ache; you got it as soon as you went directly to Kerlin's altar in the first place.

All of the lesser gods are paired with an opposite; if you do the blessings out of order, you also get cursed by their counterpart.

You may not care about a small penalty to beauty, but that doesn't mean you avoided it altogether.

(And whatever you think you saw in that cave, I'm almost positive it wasn't a black diamond. There are only two places guaranteed to have one; one of them is along the path of completing the pirate ghost's quests, and the other is in an Easter egg location you can only find by exploring, or by consulting a full map of all known locations.)

I checked my logbook for curses, and I have none.

If I got what's written here https://arcanum.fandom.com/wiki/Ancient_Gods right, you only get a curse if you attempt to get the blessings from the opposing gods, so if I went from Geshtianna to Kerlin, or the other way around, but I never gave any offering to Geshtianna.

As for the diamond, I don't remember where I got it from, so it might have been some other place than the dwarven mines, but I certainly have one. Maybe I ran into one of those places you mentioned by accident.
(edit: probably there https://arcanum.fandom.com/wiki/Ancient_Maze - the page doesn't mention neither the pirate's quest nor any easter egg, though, I didn't explore it in full, as the enemies were too powerful, maybe I'll find the easter egg part later, if it's in that one)