I don't often plant more than one plant in a pot unless it is a 20 inch pot. I may put in roses and garlic together. Many of the herbs are not heavy feeders so they don't take a lot of nutrients but, there are anti-companions that don't do well together so you do need to take account for that. I don't plant more than one thing in a pot less than 20 inches mainly because things like green onions have a surprisingly large and long root system and require the pot be watered more often.
To control nematodes with biocidal plants like sun hemp, India mustard or marigolds, these plants must be planted thickly with no weeds to act as nematode hosts. In the case of marigolds, not all marigolds work. You need to get the right marigold for the right species of nematode and be aware that marigolds can still harbor different nematodes.
Marigolds trap nematodes in the living plants, and the susceptible crop planted within a couple of months to benefit. Because French marigolds do not have as deep a root system as the African marigold or sun hemp, they work mostly at the surface. These plants help to reduce nematode populations but do not eliminate them. You will still need to make sure you follow strict sanitation procedures to prevent spreading the nematodes to other parts of the garden that do not have them.
https://www.ctahr.hawaii.edu/oc/freepubs/pdf/PD-35.pdf
https://www.ctahr.hawaii.edu/oc/freepubs/pdf/PD-32.pdf
For plants that are highly susceptible, isolation may be the best bet. Nematodes are very common here so selecting nematode resistant cultivars gets the best results and luckily there are a lot of plants bred with nematode resistance. However, if I want to plant some of the heirloom tomatoes or ginger (there are no nematode resistant varieties), they have to be planted in disease free soil or in pots with sterile media off the ground and tools and hands need to be washed before touching the soil and plants so that nematodes will not be transferred from the garden to the pot.
As for the beneficial insects, creating a habitat for them is not that hard, you want to have long lasting flowers throughout the year, a water source ( either wet leaves in the morning or a shallow tray of pebbles and water), habitat plants that insects lay eggs on and raise larvae or artificial hives for solitary bees. An small pot or 4 inch pvc pipe section make a good toad house. I plant alyssum, cuphea, finocchio fennel, marigolds, nasturtiums (seasonal), sun flower (seasonal), borage(seasonal) and lavender multifida (zone 9-11) a landscape lavender that will bloom nearly year round, unlike the edible lavenders that only bloom for a short time once a year. If you cover crop, buckwheat attracts a lot of beneficial insects and grows very fast, 6 weeks to bloom. I also let basil, onions, perilla, pretty much everything go to seed and collect the seeds to replant. Pretty much anything in bloom will attract many beneficial insects and bees. What you really need to avoid is resorting to spraying to control pests. The garden patrol will not get every bug so you will need to learn to tolerate some damage, make plants healthier by making sure they get what they need nutrients, space and water, selecting the best cultivars for your location, and culling weak plants.
Birds do eat a lot of insects but unless they have a roosting place like trees, they will try to nest in your attic. They will also eat your seeds and young seedlings and need I say anything about bird poop. If you do put out a feeder, don't do it regularly because the birds you attract will be mostly seed eaters and they will wait for you to feed them rather than catch insects. It may also attract other wildlife you may not want around.
Has anyone tried spraying a solution infused with the beneficial plant (say marigold or peppermint "tea" for instance)? If so, did it work out at all?
I've always heard bee balm was the bomb for attracting pollinators, not in my garden.