These are quite big questions and should possibly be in different threads as they do take quite a time to respond too and should generate several opinions.
Couple more questions: if it is better to feed a snake two small meals instead of one large one per week/fortnight/month, does it mean give both at the same time, or one at the start of the feeding period, and one in the middle?
Feeding regimes are really at the call of the keeper. Many keepers do it differently and even if you have a feeding 'plan' or regime these often fall by the wayside when you have a larger collection.
If you look at how a snake changes in both observed behaviour and metabolically it may help to decide the better course of action. Once fed the snakes metabolic rate will increase until the meal is digested. If you feed it twice in one week the metabolism is not going to reduce back to its 'resting' state so this implies that feeding frequently is going to be more taxing on the animals vital organs over an extended period. But feeding meals that are obviously on the 'large' side is no more beneficial as captive raised rodents contain a far higher fat content than they would in the wild.
typical feeding regimes are feed hatchies once a week. 6 months to 18 months old every 2 weeks, 2 to adult every 2-3 weeks. Adults every 3-4 weeks. In the case of breeding females this might change as they need to regain condition prior to being wintered the following season. All of this is of course as always MY OPINION.
what are materials/chemicals that should never ever be in the snake’s enclosure or in contact with the snake due to toxicity or being an irritant to skin/mucous membranes?
There are a few things to steer clear of but mostly sound logic and establishing a 'good' user friendly substrate will resolve most potential issues. Wood shavings including any ceder or pine should be avoided, (some chipped/oven baked pines I hear are ok but if you avoid them all the time it eliminates the risk). Some people use cyprus mulch as a substrate but please be careful it has no assed fertiliser before use if you choose this path.
Tape as mentioned above is a NEVER use. Avoid sharp edges on glass by rubbing them down with sandpaper. Uncoated wire mesh shouldn't be used.
Live plants don't do well in most cases and Im not convinced they add any benefit to the environment you are trying to maintain, (humidity fluctuations, water spills etc) but some people have made nice aestetically pleasing enclosures using live plants but I have also seen successes with silk/plastic plants too.
Cleaning materials can include the infamous F10 that everyone claims is soooo good but household disinfectant is good enough for the needs of any typical reptile keeper, (I used to use bleach years ago if doing a really big clean and just keep the snake out of the enclosure till it had aired) Never had any problems.
I use wood/bark I pick up from walking the dog, its no big issue, you will hear tales of bake it in the oven but all I have ever done is rinse it down. If its particularly dirty you can use disinfectant on it (I soak overnight sometimes) then leave it to dry on the patio in full sun.
I saw some nice ornaments on eBay, but they’re cheap and come from China, so I’m not sure whether they might be made from materials potentially harmful to the snake.
Reptile ornaments/household ornaments to be used by the animal or to be looked at by you? Should be fine but is this to your enrichment or the snakes? Do they actually serve a purpose? Can the animal be injured if it pull the 'ornazment onto itself? Stacks of rocks should be avoided unless they are secured. You will be surprised how strong these animals are.