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This article explains why and when rhubarb flowers and forms seeds:
https://www.gardeningknowhow.com/edible/vegetables/rhubarb/rhubarb-bolting.htm

You should remove the flowering stalk to prevent it from taking energy away from stem/leaf formation. However, it won't affect the quality of the stems.

This article will help you.

https://www.gardeningknowhow.com/edible/vegetables/rhubarb/rhubarb-bolting.htm

The safest way would be to remove the grass manually and then you can mulch the plants to keep weeds and grass down.
These links may help you.

https://www.gardeningknowhow.com/plant-problems/weeds/how-to-kill-grass-naturally-kill-unwanted-grass-in-your-yard.htm
https://www.gardeningknowhow.com/edible/vegetables/vgen/vegetable-weed-control.htm

Apply a pre-emergent herbicide around the Juniper can prevent weeds from germinating.
You will need to apply this before the weeds emerge. This can be a small time frame.
In cold climates it is early spring and warm climates it would be mid-winter.

Apply herbicides post emergent are a bit more difficult and the risk to your Juniper may be greater.

Avoid any selective herbicide that contains dicamba--Juniper are sensitive to this.
Also do not use a selective herbicide that contains glyphosate. This will kill the weeds and everything else it touches.

Read the labels carefully and make sure it targets the weed that you are wanting too.

Yes, the later harvesting resulted in hollow stems. Don't worry you can still use the Rhubarb.

Here is a link with more information.

https://hort.purdue.edu/ext/rhubarb.html

These articles will help you with the care and harvest of your Rhubarb.

https://www.gardeningknowhow.com/edible/vegetables/rhubarb/harvesting-rhubarb.htm
https://www.gardeningknowhow.com/edible/vegetables/rhubarb/grow-rhubarb.htm