1) Beside the network difficulty (i), the mining hash rate (ii), and the block time (iii), the certain exchange rates (iv), and the power consumption indicators (v) is there any other parameter that could increase the precision of a real-time mining calculator?
Well, in your "Directive" B, you already ask questions about the block reward.... You'll defenately have to take the halving dates into account, since at those dates, the reward will halve (not including the fees).
2) Are there any 'hidden' correlations between the number of transactions in a block, or the transactions fees and the actual mining capacity?
If blocks are completely full for a longer period of time, it might fuel a fee bidding war, thus increase the average fee... What do you mean by mining capacity? The difficulty is not adjusted in relation to the number of transactions or fees, nor is the block size... So i'd say that there is no correlation between them, altough i don't think this was your actual question...
3) Despite the 'spreads' between block validations, in the long term the protocol adjusts to the average 600 s block time, so I suppose that it is unnecessary - even wrong - to use a block time variable other than 10 mins. Is that correct?
That is correct, the difficulty is adjusted every 2016 blocks, the adjustment is aimed at getting the average time between 2 blocks back to ~10 minutes. If the hashrate keeps on growing (longerm), the average time between 2 blocks might actually be a very little bit under 10 minutes, because it always takes 2016 blocks before the diff is adjusted again.
A) I wish to simulate changes in the network difficulty. I think it wouldn't be wise to use a constantly increasing value. Do you have any suggestions about the actual test values and methods to calculate a realistic network difficulty scenario?
No. If the network hashrate decreases, so does the difficulty (and vice-versa if it increases). No way to tell (or predict).
B) I would like to designate theoretical dates of block reward halving in the following 5 years that are not unrealistic. What do you think? How many halvings are expected in the next 5 years, and approximately when?
This is predefined into the code: the reward halves every 210,000 blocks... ~10 minutes per block (on average), so you should be able to calculate the halving dates up to a couple days/weeks:
https://en.bitcoin.it/wiki/Controlled_supply.
C) Finally, the most uncertain variable: the exchange rate of Bitcoin. Do you think that it is a good idea to mark a certain average value (e.g. $8.000) and use it as a constant reference for the entire 5 years period?
I have no idear... Price could be $1 in 5 years, or it could be $100.000 in 5 years... If we would know the price in advance, calculations like yours would be easy and everybody would do them... If you believe in bitcoin, you can probably take a little bit bigger risk, if you don't believe in bitcoin you should play it safe... But never invest more than you can afford to lose.