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The most likely reason is that it is top heavy and the stem cannot support all the weight, so it is bending over with the overburdened load.
Thin and reduce the foliar crown by selective pruning, and consider staking the plant for additional support

Yes, this can help to rejuvenate the shrub, and get it growing like it should again. It will not hurt to aggressively prune these back to the ground if you need.Leaving some stump can help the growth process and help it to be more manageable. This article will help: https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/pruning-rose-of-sharon.htm

Your Canadian zone 5 will be the equivalent to our zone 4. They will not survive the outdoors very well. They are only suited to our zone 5, which is your zone 6, so you will be a little too far outside of their native range.

I would keep this as a container shrub, and maintain the root size to fit a large pot.

This article will help you to prune this shrub, which you will do along with trimming the largest roots out completely each year during the winter when you repot: https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/pruning-rose-of-sharon.htm

A new planting with yellow leaves; it is likely watering issues. Either too much or too little.
Daily water will be needed for the first few weeks, then you can taper off.

https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/rose-of-sharon-bush.htm

Squirrels often will nibble buds on plants to get at seeds or soft centers. Have you tried the pepper spray? There is a commercially prepared one. Spray it on all the leaves, buds and flowers. Here are more suggestions:
https://www.gardeningknowhow.com/plant-problems/pests/animals/get-rid-squirrels.htm

It is very likely that it has run out of fertilizer. This should be done several times per year to keep it blooming properly. They are heavy feeders. This article will help: https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/rose-of-sharon-not-blooming.htm