Well I'd imagine it would be like the Druid class when they're killed and when rezzed have to switch back into Bear or Cat form again. So if a Tinker is killed, they're rezzed back in caster form, and have to re-summon their mech.
I've considered the D.Va-style Tinker tank before, even to the point where a self destruct ability could be used as an AoE ability by the Tinker. I think the problem emerges when it comes to survivability of the Tinker tank outside of the mech, and how the Tinker resummons the Mech when the mech gets destroyed by the boss and not the Tinker. You could allow the Tinker to "bank" additional mechs just like D.va does in order to make self destruct into a viable AoE attack. However, the Tinker needs to be penalized for losing the mech if it gets destroyed.
For that, Self Destruct would have to have a very long cooldown. Like 10-15 minutes long.
The Tinker if blown out of the mech, would need their HP slashed in either 1/3 to 1/2 life, but should have a couple of abilities to survive, and to continue to be able to tank (at a much lesser degree). I don't think a destroyed mech should use the same cooldown as Self destruct, and a Tinker would need to have a cooldown before they can summon the mech again. In Overwatch, D.Va killing heroes reduces that cooldown time. A similar mechanic could be utilized for a Tinker. I should stress again though that a Tinker tank out of their mech should be squishy, and not capable of tanking for long periods of time.
A skilled player could theoretically never get killed if they play properly, but that's kind of true of any tanking class I suppose.
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I'm just pointing out that there's a reason Warlocks and Death Knights have highly limited numbers of summons on screen at a time, and in most cases it's just projectiles that resemble creatures.
Frost DKs summon Frost Wyrms, Blood DKs summon Bloodworms and a Ghost blade, and Unholy summons a wide variety of stuff.And what does Death Knight swarm intensity have to do with anything? I don't follow. The Blight spec wouldn't need tons of minions, I didn't propose it as the minion spec. In my mind, all specs of a Necromancer would have skeleton summoning, but the specs would specialize in different aspects of damage dealing. Blight specializes in spreading plague, specialized plague/poison minions like oozes, constructs and spiders, and using it to zone.
Just like the DK doesn't have minions in every spec. They only really have minions in Unholy spec. The Necromancer would be somewhat of the inverse where their Unholy spec would be based on plagues and DoTs with minions as utility and support while a Shadowfrost or an Anima/Blood spec could focus more on the minion style gameplay.
What does the DK have to with anything..... The DK's lore is also based within the scourge, so all of these concepts are just as applicable to the Death Knight as they would be to any Necromancer. They're the same faction, and Death Knights are actually LEADERS within that faction. I mean, Blight, Plague, Oozes, etc. are all things currently (or previously) found in the DK Unholy spec.