Acidity from decomposition & mulch Q

I've read conflicting information/opinions on the internet about decomposition and would appreciate opinions (facts are always welcome, too) -

Without regard to the type of organic matter (leaves and debris of maple versus oak, for example), as a general statement, does the process of decomposition itself affect soil acidity and if so, how?

Other topic -

I will be putting hardwood mulch around some new plantings becuase I've read that is what I'm supposed to do, but I would like to know:

(1) Is the purpose of this muching to help retain heat in the earth, and if so, does it make a difference if mulching is done earlier or later in the Fall or during a cold snap or heat wave?

(2) There will be decomposition going on which will generate some heat not solar. Would a layer of green under the mulch, such as grass clippings, be either advantageous or disadventageous?

Thank you.

To answer your second question, personally I use mulch to keep weeds at bay and keep moisture in. I suppose it keeps temperatures more constant, too. Lilies bulbs and roots need to be cool, and mulch helps with that, I hope.

As the mulch is decomposed, nitrogen is consumed... I think that's more of an issue, but now that you mention it, all the mulches I have used contribute to soil acidity because they are acidic to start with - pine bark, hardwood, pine needles,etc. I don't know if decomposition adds to the acidifying effect of these materials. A lot of my plants are acid-loving, so for me it's not an issue.

People mulch in winter to protect plants from frost heave (getting pushed up by freezing & thawing cycle). My understanding is that this mulching works better late, after the ground has frozen to keep it frozen. If you have solid snow cover, that will do the trick.
Mulch in spring & summer is for the reasons claypa mentioned.
Someone here explained to me that nitrogen draw is not that big a deal and it would be less so with hardwood mulch (slower decomposing)

Mulch serves several purposes - it smothers for weed prevention, it helps reduce wide fluctuations in temperature and moisture conditions. (Helps retain warmth in cool weather, helps cool soil in warm weather... keep moisture from evaporating - helps retain moisture but it can also absorb excess moisture...)

As for nitrogen consumption - it will only consume nitrogen in the very top layer of soil that it is in contact with, probably a bonus for weed suppression. Plants in mulch have roots that go much further into the soil, which is where the plants get their nutrients.

A layer of greens under hardwood mulch - will help the much decay faster - creating good organic material for the soil.

As for acidity - as things decompose - the pH goes towards neutral. So adding compost or decomposed organic matter has a tendency to neutralize pH. As mulch decomposes - its the same process. And typically in a mulched bed - we are continually adding layers of mulch - the old mulch decomposes and becomes organic matter added to the soil, the new mulch remains on the top of soil providing weed, moisture and temperature control.