Gundam Thunderbolt Timeline?

[MythSearcher]

Wouldn't they just need to buy Dai Nippon Kaiga in order to fix that problem?

Yes and no.
If they want to be 100% sure they acquire the rights to Sentinel, that is the most affluence way to do it.
Worst comes to worst, the hostile takeover path is the last ditch effort they can do, which is pretty unlikely for a single title. Considering Dai Nippon Kaiga's isn't really overlapping a lot of business with Bandai, this method is VERY inefficient. Even if they can just purchase the business of Model Graphix isn't worth it.(and since their own Degeki Hobby didn't work, I especially don't think they want to spend a lot of money into purchasing a publication business soon)
Technically they can just buy the rights for Sentinel without buying the whole company, given that the company is willing to sell it. They probably won't but their relationship with Bandai surely improved a lot, so we can see they have regular collaboration, the main problem will be that everything in Sentinel will cost them a sum of money, which is the leftover price the have to pay for their arrogance back then. But seriously, that price will still likely be very small compared to buying the rights off of Dai Nippon Kaiga's hands in the foreseeable future because it also benefits them to sell it cheap to Bandai.

But an anime of Sentinel is very unlikely because 0083 is Sunrise/Bandai answer to Sentinel, they are not likely to do it again.

[Mafty]

I often wondered if 0083 was an attempt at doing a non-copywrite infringing version of Sentinel , as it shares some similarities, (ie More obviously Military themed, Older characters, a lot of technical detail, and several big battles around asteroid bases). I'll admit I don't really know how Copywrite Law and Licensing work (especially in Japan where they'd probably have different laws),but would buying a company really give you ownership of their products? For example Sunrise Beyond is made up of a sell off of Xebec; however Sunrise dosen't own Xebec's catalogue of shows(which is kinda a shame since Sunrise specializes in mech shows and Xebec has several notable ones, such as Nadesico, Zoids and Fafner), instead the shows rights were sold to Production IG.

[Seto Kaiba]

I'll admit I don't really know how Copywrite Law and Licensing work (especially in Japan where they'd probably have different laws),but would buying a company really give you ownership of their products?

Assuming the company being purchased is purchased in its entirety? Yes, it would. That purchase would make the purchaser the new owner of its assets, its intellectual property, and also its liabilities and any or all contractual or legal obligations the purchased company still had.

Some sell-offs, like Xebec's, are partial as the parent company wants to retain some or all of the subsidiary's assets and/or intellectual property.

For example Sunrise Beyond is made up of a sell off of Xebec; however Sunrise dosen't own Xebec's catalogue of shows(which is kinda a shame since Sunrise specializes in mech shows and Xebec has several notable ones, such as Nadesico, Zoids and Fafner), instead the shows rights were sold to Production IG.

Sunrise did not acquire Xebec in its entirety. Xebec's parent company, IG Port, sold off the studio itself, but not the post-production teams (which were incorporated into other IG Port-owned studios) or its intellectual property which was transferred to parent company Production IG.