Aww Polaris
you have a really tough job with the two of them and you are doing it wonderfully. They are lucky to have you. All of us do things we regret in really stressful moments. Please don't be hard on yourself.
It really does seem like Clara has had a developmental spurt lately. So exciting, but I'm sure it is making sleep tough.
Noelle, sorry about the nap transition!
I'm quite sure that's exactly what's going to happen with Munchkin when it's time. I can't believe C never slept more than 45 minutes as a newborn...I can't imagine what you must have thought and felt!
Matilda, welcome and sorry you are here
6 months is exactly when most of us found this thread, if you look back. It can be a surprisingly tough time for some babies.
Here are my ideas for you:
1) Nap transition. Can you post your daily routine? Things are often changing around that age and longer wake times can help them sleep better. Won't teach your LO to self-settle, but at least maybe you won't have as much trouble putting him down or have to wake up as much.
2) Shh pat is really useless for many babies, so don't worry about that if it didn't work for yours. Have you heard of or tried the No Cry Sleep Solution gentle removal/Pantley pull off technique (it goes by a lot of names)? That is a gentle solution that could work for you. If it doesn't work then you may have to decide on either WIO or letting him cry.
3) If WIO ends up seeming right, is bedsharing an option you'd be willing to explore?
4) Does LO have any signs of allergies or food sensitivities? This is probably not the culprit. But sometimes food sensitivities do disrupt sleep. Just because you said that the worse sleep coincided with the solids. Again, unlikely, and there are plenty of more likely explanations at this age, but possible.
Also please don't feel badly about what you tried at 4 months. Some experts advise not to let them cry at that age, as you know (it's repeated all over BnB -- not until 6 months), so of course you did what you had to do. Lots of babies learn to settle without those things on their own. It's not your fault that yours didn't.