Numerounius

I am not sure but it could be tileset related. There are some tiles that are really rare based on how they be generated. So I wonder if that is translated to the three different boss tileset spawns. However, I could be wrong with them all having the same hitboxes with it bordering the void and T connectors to the adjacent tiles for the other ends.
E:Nope the tilesets are different yep, they will have an influence on which boss spawns most often unless they changed the tileset predictor to account for boss spawning tiles first or something like that.

"The Gotva Prime Rifle will be exclusive for a limited time, but players who miss out on the Drop can earn it in-game at a later date." Directly from the tennocon page, can't find the exact page right now but this quote exists from it.
I too, am hoping that it comes sooner rather than later as I wasn't able to get it either and is my only missing MR right now.
And the person relates it to the Athodai which is Not a Baro weapon.
 
Athodai tennocon August, released into railjack in March.
Vastilok was released through tennocon June 2021 and first Baro appearance was in October so that is a 4 month span.
Vericres released July 16, first baro appearance November.
Gotva prime, tennocon August 26. . ??? Which is much later date than the previous two tennocons.
 
So if following the recent releases and time frame I would leaning on first baro of next year or in the first mainline content piece of that year but I would not rule out an "oh we forgot" and release it in May next year situation as that is also possible since DE doesn't have a set frame/cadence for when weapons are supposed to be available.

They are consumed but this is something that clashes with what DE has said about the mechanic/interaction. See below, the main patchnote reads like it is patched to no longer be working yet goes on in the footnote to state it is still possible yet you can get them back via contacting support. Really don't know why they wouldn't just automatically refund it or prevent subsuming the frame in the first place and instead just making the "solution" being not changed and just a popup screen which is already there for subsuming meaning it wouldn't have the desired effect of preventing people from doing it and then requiring contacting support afterwards.

The best candidate is probably going to be the new Ceramic dagger incarnon weapon, that has a perk "gun and blade" that allows you to stack 100 initial combo with primary kills which when combined with initial combo mods through corrupt charge, covert lethality and a riven with IC on it can get pretty high, not sure where the limit is but it is pretty high. And since it is all initial combo it won't go away when you heavy attack.
There might also be other options that might be worth looking into such as the Tenet Melees that can store combo when holstered or gunblades that don't quite meet the full bill but are able to score huge heavy attack hits. I am not sure how familiar you are with modding around heavy attack/combo multis since there are methods to build up combo multiplier quickly that allows for a dedicated heavy attack build.

Relevant snippet out of patch notes:
If you do not own any of the variant you get default.
If you own one it takes it.
If you own two or more different from the same variant it takes the highest classification so sadly it will always take an owned prime version.
And then I'm not sure how it takes it for 2 or more of the same one so I would assume it would take your oldest similar to how helmith decides your replacement frame.
 
So in your situation, if you have a modded regular and a basic prime warframe. It takes the prime since that is what you own and is the most powerful version. If you sell it, it offers you the normal variant and not the prime since that is your highest owned variant.
 
Sadly this aspect of selection along with randomization and trying to pick exactly which loadouts you want picked has been heavily requested since Duviri's launch and I think they mentioned something about looking into it on the devstream. I am not sure how long we can expect to see changes in the game but if it was up to me I would advise against selling your prime warframes.