1) Should be OK to trim back milkweed but you will probably lose all this years blossoms.

2) Datura seed pod should be allowed to ripen on plant until it starts to split open.

Then allow it to dry indoors til next spring. the seeds need to mature before planting.

2) Datura - The seeds are inside that pod... hundreds of them. Wait until that pod turns tan/brown. Then you can crack it open and harvest the seeds and plant (or save until next year to plant).

4) Purslane - Not a direct answer to your question, but might be helpful: Purslane and portulaca are related but very different

Carol in Jacksonville