1: I year is good. 5 years can often be better!
2: Yes, rack before ageing/conditioning. Once is enough, just to take the young mead off the precipitated yeast.
3: No
4: Neither is possibly better. But you are the brewer and can adjust as you prefer.
5 No. Pasteurising is probably a bad move. Adding chemicals can be done - but why? Mead is usually sufficiently alcoholic to be stable. Better to use a yeast that has the desired degree of attenuation with an initial surplus of honey. One can use a non fermentable sweetener (lactose, stevia) but that would not be a very authentic mead.
6.Good question.... The usual answer is to leave in a cool dark place like a cellar. But a shaded place that has least temperature variation and does not freeze will do. A refrigerator can be used but it is not optimal. I have conditioned beer in a fridge for 18 months just to see what happened. The result was possibly slightly worse than conditioning at room temperature
Answer to comment below:
There is no particular need to use one type of yeast. Some are happy with what they have and others are forever looking for the perfect yeast. Different yeasts ferment to different levels of alcohol. Attenuation is a measure the yeast ability to convert sugar to alcohol. Or perhaps the level of alcohol a yeast will stop fermenting at. Low Attenuation = lower levels of alcohol in final brew. High attenuation yeast = higher final level of alcohol. But choosing the yeast and adding an excess of honey for that level of attenuation one can vary the level of alcohol and level of sweetness in the final product. A dry mead is produced where there is just enough honey sugar to ferment to the level the yeast stops at. A sweet mead is made by adding more honey than the yeast can deal with. So some residual sweetness is left when the yeast has reached its preferred attenuation level. In this case it seems you need a lower attenuating yeast than the one you currently use. Alternatively you can use a non fermenting sweetener (like stevia) but some consider that cheating...
Back sweetening is not bad it is just difficult to manage for anything other than immediate consumption in a "live" brew. If the mead yeast is still active then adding honey will cause it to ferment further. You could pasteurise the mead or add preservatives or growth inhibitors (like Ksorbate) but that would be a personal choice. I would prefer to arrive at the preferred sweetness by a more "organic" method such as choice of yeast. YMMV
