Wet cloth.
Club soda can be used to remove ink marks. However, it will work better if it is combined with baking soda. Together they will bubble up quite a bit, and work to completely remove the stain.
Iron marks burn the fabric, which damages it, so it may not be removeable. If you can get the black mark off, the fabric underneath it will be weaker than it originally was. To try and remove the stain, scrape off any charring, then pretreat the stain, rinse it, and then wash it as you normally would.
You cannot. A glue stain, or any stain will show through clear varnish. It is always better to try and remove all stains or marks before varnishing. Almost every glue has a solvent that will remove it.
It is highly recommended to treat the lipstick stain as soon as possible. Apply a before wash stain remover and rinse the shirt in warm water, being careful not to rub the stain.
I would remove the stain from the entire piece (leg or top) and then carefully re-stain it properly.
Stain stick
black skid marks
Not all stain removers are acidic. Some stain removers may contain acidic compounds, such as citric acid or acetic acid, to help break down stains, while others may rely on enzymes or surfactants to remove stains. It depends on the specific formulation of the stain remover.
You are probably not going to remove the "stain" from the cover. It has been bleached of all pigment.
There really isn't a way to remove a paint thinner stain from wood furniture. You must repaint or re-stain the furniture.
To remove a Pentel pen stain from clothes, you can try rubbing alcohol or a stain remover directly on the affected area. Gently blot the stain with a clean cloth until it starts to lift, then wash the garment as usual. It may take a few tries to completely remove the stain.