Itinerary Help for May 7 - May 17

Congrats!
I can't speak to the seasonal aspect, but one day in Zermatt is generally ill advised because the clouds can obscure it and then you've only had a day, and after all that time on the train getting there.
Also, how did you plan to get from Zermatt to Annecy? Unless there is a scenic train that I am unaware of, that stretch seems unrealistic.
That time of year should be really exceptional on the lake, so I would put some of your days at the end together there. Lausanne-Vevey-Montreux puts you in reach of Geneva airport, in addition to being lovely. I'd add a day to BO.

That is definitely a concern of ours for Zermatt - but I've also read staying there too long during the time we will be there is also ill-advised!

We would have to go back through Geneva to get to Annecy (I'm on the fence about keeping this in the itinerary).

Thanks for the advice - I'll look into those areas close to Geneva.

Yes, look at train times on the SBB website.
Montreux area in May should be flowers galore--read about the narcissus here: https://packedagain.com/things-to-do-montreux-spring/

Another resource is to look at the SBB rail map. (It blows up as big as you need it). As you can see, it is quite a complicated path to go from the Jungfrau region to Zermatt; and as already noted from Zermatt to Annecy. Basically two of your limited nine days will be transiting through various stations and connections. I would cut both Zermatt and Annecy, and add more time in the Jungfrau region and the Lake Geneva region.

I think the Zermatt/Annecy/Geneva portion is going to be a bit much, so you probably want to cut one of those otherwise it’s going to be a blur for your last few days - not relaxing at all.

In regards to the Jungfrau Region, the Train to Murren is closed May 6-31 to install the new train cars, so to get there will require a bus connection to Stechelberg. No big deal if you plan to stay up there the whole time and just do Gimmelwald, Murren, Schilthorn, Walking in that area, but if you want to be coming/going, Wengen might be better for your dates.

You might be able to do the North Face hike from Murren - tbd on snow melt. Or the Mountain View trail. For sure you can walk from Murren to Gimmelwald on the paved road path.