BDSP faithfully remakes Diamond and Pearl's evolution system. All 29 cross-generation evolutions from Gen 4 are available, and the Grand Underground adds encounter rooms that give earlier access to some chains. Garchomp's chain starts with Gible in Wayward Cave, one of the earliest pseudo-legendary chains in any game.
Yes, all 29 of them. Electivire, Magmortar, Rhyperior, Togekiss, Gallade, Froslass, Dusknoir, and the rest are all obtainable. BDSP is a faithful remake, so the evolution methods match the originals exactly.
Absolutely. The Grand Underground hideaways spawn Pokemon from different regions, including some that were postgame-only in the originals. You can find things like Larvitar, Magby with its evolution item, and Elekid earlier than in DP. It makes completing cross-gen evolution chains less of a postgame grind.
No. It's a strict remake. Every evolution method matches Diamond and Pearl exactly: levels, stones, friendship, held item trades, location-based, and move-based. No Mega Evolution, no regional forms, no new mechanics added.
The Grand Underground is your best source. Dig into the walls and you'll find Fire, Water, Thunder, Leaf, Moon, and Sun Stones as rare spawns. A few stones also appear as overworld pickups on routes. The digger brothers near the entrance to the Grand Underground can occasionally find them too.