Is it safe to compost and grow plants in the above mentioned mediums? Are all HDPE food grade?
HDPE is safe for these applications. BPA is bad about leaching.
HDPE will be preferred for your, particular, need.
If I sieve my compost can I use it for growing seeds?
No, it will contain too much nutrients. You will want to start with a seed starting mix with low nutrient content to avoid burning the young plants.
Once they are established, then you can move them to a half compost, half potting mix blend.
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/planting-in-pure-compost.htm
https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/soilless-seed-mix-info.htm
I gathered brown leaves and mixed them with some kitchen scraps. Watered the whole thing and left it 1.5month alone while turning the pile. Can i now plant seeds directly in this pile with little to no soil used?
No. A few things will happen when you do this.
The fungi and bacteria responsible for creating the compost will not discriminate between the organic material and the seeds.
If they make it through the decomposition phase, which is unlikely, the nutrients from compost alone will burn the young plants.
Compost dries out really fast outside of a container that is made for composting. The conditions for composting and the conditions for plant growth are highly incompatible.
Here is a composting guide that will help you to create compost, which should be used sparingly WITH soil:
https://www.gardeningknowhow.com/composting/basics/ultimate-beginners-guide-composting.htm
This article will explain why compost needs to be cut with soil:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/planting-in-pure-compost.htm
Kit scraps and outdated box cereal some old vegetables; is over 170 temp too hot?
These ingredients can be a small part of a balanced compost. The temperature will maintain itself in a properly set up compost.
This guide will help you to compost properly: https://www.gardeningknowhow.com/composting/basics/ultimate-beginners-guide-composting.htm
I live alone and don't always finish any leftovers, especially after my son has been here, and would like to know if I can compost more than just vegetables. I already compost leaves, grass clippings and branches/wood for free topsoil for my yard, can that be used in a vegetable garden as well or just keep it for lawn and flower gardens?
Meats are not recommended in the compost pile. You can use your finished compost anywhere.
Here is more info:
https://www.gardeningknowhow.com/composting/basics/starting-compost-pile.htm
I add grass clippings which become very hot but cools within a couple of days, on turning it looks as though I have mold. Can I add sawdust for brown waste, my heap is too old.
Yes you can use sawdust and/or newspaper. These articles should help further:
https://www.gardeningknowhow.com/composting/ingredients/browns-greens-compost.htm
https://www.gardeningknowhow.com/composting/ingredients/what-can-you-compost.htm
I live alone and my coffee grounds, veg & fruit scraps take a while to accumulate. I store them in a covered 5 gal bucket, it takes 4-6 weeks to fill and then add to compost pile. By then everything is well rotted. Is this good or bad? OR Should I just add the weekly scrapes on a weekly basis? No picture
These scraps should go directly into the compost pile. Two piles may be necessary to allow one to compost, and one to actively add compost to.
Letting it rot in the container, first, may introduce pathogens that you don't want, and increase the risk that they will not die off in composting.
This article will help you with proper composting technique:
https://www.gardeningknowhow.com/composting/basics/ultimate-beginners-guide-composting.htm