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This article may help you decide whether to pinch your sweet pea or not
https://www.gardeningknowhow.com/ornamental/flowers/sweet-pea/pinching-sweet-peas.htm

Yellowing leaves can indicate watering issues, disease, pests or nutrient issues.

Inspect the plants and leaves for signs of insects or markings.
Check the soil. Is it well draining? Lack of water or excess moisture can be an issue.
These articles will refresh you on the care requirements.

https://www.gardeningknowhow.com/ornamental/flowers/sweet-pea/care-of-sweet-peas.htm
https://www.gardeningknowhow.com/ornamental/flowers/sweet-pea/sweet-pea-flowers-falling-off.htm
https://www.gardeningknowhow.com/ornamental/flowers/marguerite-daisies/marguerite-daisy-flowers.htm

A couple of light feedings, half strength, a year with a multi-purpose fertilizer or once in early spring with a slow release multi-purpose fertilizer is all this plant needs. Too much fertilization will encourage plant growth, putting strength into growing new foliage leaving little for new blooms.
Sunlight will also affect the plants blooming. This plant needs at least 6 hour of direct sunlight a day to bloom well. This is especially true in the spring and early summer to start a good blooming season.
Once established, which your plant should be, the Sweet Pea Shrub only requires moderate to little water. Too much water will slow blooming and in time too wet a soil can kill the plant. Many times too much water will show up in a yellowing of the leaves.

https://www.gardeningknowhow.com/ornamental/shrubs/sweet-pea-shrubs/growing-sweet-pea-shrubs.htm

Yes, you can mix the neem oil with water plus a little soap to allow the oil and water to mix. Add approximately four teaspoons of neem oil per gallon of water, plus a few squirts of mild dish soap. Simply shake and spray.

Also, it's best to apply in the early morning or evening to avoid burning the foliage in the bright sunlight.

I have never taken cuttings. Wait until the seed pods begin to dry and fill out and there is evidence of seeds inside. Open one or two, you are looking for hard, dark brown / black seeds with possibly a white dot. If the seeds are green then you have collected them too early. You can risk planting a few at the end of October / November, but I always wait until spring and keep the seeds dry until then. Before you sow them, soak them overnight in a cup of water to help germination.

Failure to produce flowers is often a sign of excessive nitrogen in the soil. Sweet pea is especially sensitive to this problem because it is in the legume family and can fix its own nitrogen in the roots. It can also be caused by stress from high temperatures.

https://www.gardeningknowhow.com/ornamental/flowers/sweet-pea/sweet-pea-flowers-falling-off.htm

This article is about beans but it's the same idea:

https://www.gardeningknowhow.com/edible/vegetables/beans/get-bean-plants-to-bloom.htm