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The best time to prune Clematis montana is immediately after flowering, typically in late spring or early summer. Start by removing any dead or damaged stems, cutting them back to healthy growth. Next, thin out the plant by removing some of the older, woody stems at the base, allowing younger, more vigorous growth to take their place. Trim back any excessively long or unruly stems to encourage a more compact and tidy appearance. It is important to avoid heavy pruning in late summer or fall, as this can remove the flower buds that will bloom in the following spring, so I would wait until then. Regular pruning will help keep it in check and ensure a beautiful display of flowers year after year. These articles will help:

https://www.gardeningknowhow.com/ornamental/vines/clematis

That will be a group 1 Clematis. This means that it blooms on old wood, so pruning will not be necessary unless it gets unruly or to remove dead vines. Winter care should be taken. This is only barely hardy in your area, and some cultivars will be a little more sensitive than others. You may need to mulch extra heavily in winter, and remove the mulch in spring. I would expect some dieback each year, but this should not be too bad.

https://www.gardeningknowhow.com/ornamental/vines/clematis/growing-clematis.htm

https://www.gardeningknowhow.com/ornamental/vines/clematis

It looks like it has been through some freezing weather. Princess Kate clematis is deciduous and should lose its leaves in the fall. In the spring, cut back the old vines above a node to 15-20 cm. (6-8 inches) from ground level. New stems should grow, followed by flowering.

If it is wilt, the roots usually survive and send up new shoots in spring.

https://www.gardeningknowhow.com/ornamental/vines/clematis/clematis-wilt-treatment.htm

I see what you mean. There is conflicting information about growing clematis from seed, but all the references I see say it takes from about 6 months to 3 years, so you may have given up too soon. Stratification seemed to be key - planting the seeds outside in the fall and letting them sit dormant in the winter cold, then sprout with other perennials. You can imitate this cold period in the refrigerator for three months. Keep the seeds moist in compost or a seed starting medium.

I saw Montana seeds for sale by a couple vendors and both said to use the stratification method.

I had the late summer blooming sweet autumn clematis, which is a rampant reseeder. It is considered invasive in some states. So some clematis grow easily from seed.

Look for longtime retailers online and read their "about" information. You can also ask family and friends who they order from. We don't sell any plants or garden products, but we do offer free gardening information. These articles may help you choose a variety. Your local garden centers should have some for sale.

https://www.gardeningknowhow.com/ornamental/vines/clematis/choosing-different-clematis-vines.htm

https://www.gardeningknowhow.com/ornamental/vines/clematis/growing-clematis.htm

https://www.gardeningknowhow.com/ornamental/vines/clematis/pruning-clematis-vines.htm

https://www.gardeningknowhow.com/ornamental/vines/clematis/fall-flowering-clematis.htm

https://www.gardeningknowhow.com/ornamental/vines/clematis/summer-blooming-clematis.htm

https://www.gardeningknowhow.com/ornamental/vines/clematis/clematis-bloom-times.htm

I wouldn't worry too much. These can be invasive in some areas, and tolerate heavy pruning, though it isn't ideal. This can effect blooming next year, depending on the type that you have.

https://www.gardeningknowhow.com/ornamental/vines/clematis/pruning-clematis-vines.htm

https://www.gardeningknowhow.com/ornamental/vines/clematis/growing-clematis.htm