with out home mechanics is revealing Shadow of the Past during the encounter phase the only way to get Nemesis cards brought in?

Currently, yes.

thank you for that. might just make my own home brew mechanics to get them in more often.

56 minutes ago, azeronbloodmoone said:

thank you for that. might just make my own home brew mechanics to get them in more often.

You could start your game with your nemesis cards in the Encounter deck already. I've done that. Optionally, what I do when I draw Shadow of the Past in this case is I put the nemesis cards into play directly from that deck (or the discard) instead. Shadow of the Past will surge only if the nemesis was already in play.