This non-canon setting travels parallel to canon, though it may differ from it in minor ways, or we may set down in hard fact, what TPTB are vague about. Mostly, the biggest difference is that the setting is bigger. There's more of everything. More mechs, More vehicles, more units in general. Succession Wars era tech is still around, and is usually found along with primitive mechs and converted industrials as planetary millitia. most planets ought to be able to field anywhere from a lance to a company of older tech units. As such, there's more factories and companies to run them.
Unless something definitive in canon comes out to contradict it, the passage about a part of the surviving wolverines being a part of oldschool comstar and the Word of Blake is true, but unverified as far as most characters would be concerned.
There are incredibly rare mechs, there are mechs you could go a lifetime without seeing, but its difficult to say that any given design is thoroughly extinct, never to be seen again. Even if a design is out of production, and no operational models are aware of, its pretty likely that someone, somewhere, in the whole of the inner sphere, has the data needed to re-create it.
the War of 3039, and the return of technology follows more closely to the 20 year update and the allusions in Blood of Kerensky, rather than the changes brought about by CGL's coverage of the era. Experimental versions of the high end technology may have been available to those in a very rarified circle, but most mechwarriors wouldn't be familiar with ER Lasers, Double Heatsinks, and the like.