Just ran a 1p solo playtest. I was able to finish the game, including setup and put-away, in 45 minutes total. I still think that 100 minutes for the 4p game is pretty realistic. A few observations:
First, in the last couple of sessions, the temple has been a bit too easy, because the grail room happened to be in very close proximity to a font room. I think that either the rooms all need to have the same back-printing, or the temple setup needs to be more scripted so feature rooms are further apart, or there need to be 5-6 features instead of 8, or some combination of these. It's just a bit too easy to get lucky -- in this game, the player had good temple location and true grail knowledge, and no features knowledge, but the 2nd feature he tested was the grail and the third (right next door) was the font. And of course, the grail he picked on a 50-50 shot was the true grail! It's just more exciting when it's a bit more of a nailbiter.
The enemy progress track doesn't quite get all the way down in the 1p and 2p games, so it may be necessary to shorten that track a bit for those player counts, which is easy enough.
There still isn't quite as much globe-hopping as I'd like; it's usually easiest to just visit cities more or less in order. The removal of the "relics" from the game, and the mini-race element they brought, is probably most to blame for this. To be sure, the possibility of cards disappearing due to the action of operatives creates some urgency that motivates movement, but still, it's rare that you revisit a city you've previously visited, and it's rare you go screaming across the globe to get to a theme card ASAP, because chances are it will still be there by the time you get there.
It may be advantageous to split "receive a reward" and "visit a theme card"; maybe receiving a reward can simply be an action unto itself, so it still has a cost.
The enemy operatives, who go around removing stuff from the board, don't exactly work in the 1p game, because they're always chasing the players around, so chances are they removed a reward token or theme card that you've already used. Not sure how to make them move ahead of you, but it may just be that we need to add a reason for you to want to go back to visit a theme card a second time.
One simple way would be this: most theme cards give you bonus check marks if you discard the correct Item card, but if you don't have it, you can't get the check marks. If this changed to "the bonus check marks are only available the SECOND time you visit the card, but you don't have to pass a challenge when you do it", then this would give you a reason to move around more, particularly if you're limited to only carrying one item card at a time. I don't really like this rule, it's kind of fiddly and annoying. But something like it might be a good idea.
Another mechanism we can use is Event cards; these can be thematic, and they don't add extra rules. What triggers an event? Probably drawing an "event chip" from the cup, or maybe certain spaces on the progress track. Maybe the events just move the operatives around a bit to shake things up, but maybe other things can happen as well, like moving theme cards around, although it's a short enough game that you don't want the route planning to become too chaotic.