No, use that connector and solder SDA and SCL to MSIO and MOSI
Your drawings is correct, but at startup the MOSI and MISO are in input mode and stay that as long i don’t use the SPI functions so the is no problem. To be sure, it’s always possible to make the connection with a 200ohm resistor to limit the current in case something goes wrong.
Correct, 200 ohm Wil protect it. The check of the SPI DDR registers is to guaranty no software is trying to modify these MISO MOSI pins from inputs into outputs. It’s not stupid my friend. By using the SDA and SCL pins on the ICSP header I can still use the TWI module of the ATmega and program it as a I2C slave.
Try-state means the connection to the external circuitry is open or disconnected. I doubt such condition is allowed in TTL circuits.
That does not mean you have the right to risk damaging an otherwise healthy Arduino board by experimenting with something you are not sure about.
Of course you have the right to risk damaging your Arduino.
It's YOURS, and it probably wasn't expensive enough that you need to treat it like Grandma's Lenox Crystal wine glasses!
@westfw
Then why was I instructed for 15 years by my former company, Schlumberger, to “save yourself and save your equipment”?
People who work with hardware often care for their equipment as they would for family members, and the same is true for software engineers and their systems. There should be no compromise when it comes to avoiding damage to equipment or software by failing to follow the Safety Precautions clearly stated in the company manuals, regardless of how expensive or inexpensive the equipment may be. 
Were they talking about $5 Arduino Clones, or even $30 Genuine Arduinos? I doubt it.
Besides, that was THEIR equipment, not YOUR equipment!
The whole thread is a nonsense.
There are no SCL and SDA pins on ICSP header.
Could be a problem depending on the value of the I2C pull-up resistor you have. You are now creating a voltage divider
I agree
It is not clear to me what @wogoos is intending to do
I had assumed initially that they wanted to hijack the 6 pins of the ICSP connector in order to plug in an external I2C device by cross connecting them with the I2C pins on the reverse of the board.
However, that seems not to be the case
Which might have been
The Nano has a nifty connector on it for ICSP. I want to use that connector for different signals and voltages. Can I leave MISO and MOSI connected but unused without issue?
and the answer had been given - of course you could leave them connected, you just won't be able to use them.
Since changing the default startup pin mode INPUT constitutes use, all you need to know has been said.
Just don't use MOSI and MISO.
If this interpretation is correct, it raises the interesting question: how are you going to get the 12 volts on there and how will you do the interrupt line and why don't you just do the same thing for SCL and SCK and have no questions for the forum?
a7
BTW, you could also use the ICSP connector as-wired, and run a software bit-banged I2C implementation on those pins (using them just as digial IO.)
ha ha that is what I was thinking to I hope woke as not entered the electronics. You are right I can blow up any electronics I like, although that normally not my goal.
Yes my friend that what I plan to do. Connect the MOSI and MISO to the SLC and SDA pins Arduino Pin A4 A5
That is correct, using a bit bang library. I looked at that option however there is no I2C slave Bit bang library, the ones I found are supporting only I2C master mode.
Reading my friend, I never said that I said that I wanted to bridge A4 A5 to the MISO and MOSI pins of the ICSP header. My question was if that was possible if I check on the DDR states of these GPIO’s. One person confirmed to me that it can be done. Many others are either not reading or responding to a question I did not ask.
But you also said
Those two statements seem to contradict themselves
That's one of the big problems with the Forum, many people will not spend the time to read all the posts from the beginning and only respond to your last post. It's up to you to separate the chaff from the wheat
So what is the contradiction, then? I have no idea.