Hi, I will try to answer your question as well as give some advice.
mono/stereo issue: most modules that have stereo outputs will go into mono if you only use one exit. Often the left output. Check the manuals how that works for the individual modules. So you can likely patch Monsoon Left into Grainity. and have all of the sound that was coming from the left and right input. Regardless of how you do it, after the mono Grainity the information will be mono. Running stereo into mono will always result in a mono (=both speakers the same) sound. The cable that you show has a stereo end, your mono modules will probably get no sound from that. I accidently bought some stereo big jack cables in the past and it often causes problems with Moog gear, with my bass guitar, with my effects etc. I'm sure it will be the same with 35 mm jacks.
I see you have three mults. I think you can probably suffice with two and I would also spread them through the system instead of keeping them in a corner for maximum effectiveness. The mults could get your double outputs into one input if the left mono implementation mentioned above is not in the module.
If you want a versatile and cheap LFO, the Eowave Zone B.F. Black Edition is now on Modular Grid for 46 €. That's half what I paid secondhand and frankly a great deal. Run a LFO at audio rate speed in any oscillator or VCA and you'll get interesting timbres. Please note that I'm not the seller and not responsible for the sell.
You have quite a bit of mixing modules at the end there. Mixers are sometimes optional. There is no problem going straight out from a VCA into your DAW/Audio interface if you keep the levels alright. Turning on 'pad' on your audio interface will keep the signal from overdriving. So maybe you can take one of those mixers out. Keep in mind that a Quad VCA is often also already a mixer, there is no rule on what the spot is where you take sounds in and out of your eurorack system. More VCAs is probably better than more mixers.
If you're planning on using the eurorack with the DAW always, then those two multi-effects modules might be a bit unnecessary and boring. You can apply simple end-of-chain effects in the DAW. I think it would be more fun to focus on eurorack effects with lots of modulation options, that would take advantage of the eurorack 'way of working' better.
Good luck!