This answer addresses your second question, regarding the specific case of SGA 5. In short: the contents of **SGA 5 Exposé II** can be found in Deligne's "**SGA 4 1/2**". Also, see my other [answer](https://mathoverflow.net/a/480233) about the original Exposé.

According to Grothendieck's "Récoltes et Semailles", SGA 5 was totally butchered by Illusie, in a combined effort with Deligne so that it looked useless in comparison with SGA 4 1/2 (which wasn't a true seminar, and stole some of the missing exposés from SGA 5). This is also the reason why SGA 5 was the last to be published.

About the lost exposés: 

- There was a group of introductory exposés about the relations of SGA 5 to other contexts and about the philosophy of six operations. Illusie had them, but he sent them to Grothendieck and I don't know where they ended up. I personally think they are in the [Grothendieck Archives][1] (Cote nº33) You can read more about them [here][2].
- Exposé II, as I said earlier, was reworked by Deligne and included in his volume about Étale Cohomology. The original exposé is kept at the IHÉS and can be read online (see my other [answer](https://mathoverflow.net/a/480233)).
- Exposé IV, about "The cohomology class associated with a cycle", was going to be redacted by Deligne, who instead included it in SGA 4 1/2, chapter 4. This theme also included an étale version of homology, with a formalism about the homology class associated to a cycle. According to RéS, these ideas were published by Verdier in an article with the same name. You can read it here: <cite authors="Verdier, Jean-Louis">_Verdier, Jean-Louis_, [**Classe d’homologie associee à un cycle**][3], Astérisque 36–37, 101–151 (1976). [ZBL0346.14005](https://zbmath.org/?q=an:0346.14005).</cite>
- Exposé IX was about Serre-Swan modules and was published elsewhere by Serre ([Linear Representations of Finite Groups](https://doi.org/10.1007/978-1-4684-9458-7)). This one was published by the IHÉS and can be found in some libraries.
- Exposé XI was called "Computation of local terms" or something like that, and was substituted by exposé III-b. Apparently, Bucur wrote it and sent it to Grothendieck, who somehow lost it.
- There was also a last exposé about open problems, which Grothendieck talks about in RéS and which notably contained a conjectural "discrete Riemann–Roch theorem", later referred to as "Grothendieck-Deligne conjecture" and studied by [MacPherson][4]. He also mentioned a trace formula modulo $p$, which was treated by Deligne in SGA 4 1/2.


  [1]: https://grothendieck.umontpellier.fr/archives-grothendieck/
  [2]: https://mathoverflow.net/a/460231/385781 "Answer to 'Original reference of six functor formalism?'"
  [3]: http://www.numdam.org/item/?id=AST_1976__36-37__101_0
  [4]: https://www.jstor.org/stable/1971080 "Chern Classes for Singular Algebraic Varieties"