I am sorry, but I still dont get this.
Your oldest child is 15 years.
Jen was your girlfriend from before you got married. You have been married 8 years.
You work in the same place as Jen.
Your wife started acting strange in October.
Your wife left mid November.
Jen discovered you were not coping in work.
You have only had a working relationship with Jen until after your wife left.
A month ago.
Jen says she loves your children. Really?
Did she start loving them this last month? All 5 of them?
Last week, after your wife had been gone 3 weeks Jen started talking about "us". You are considering hooking up with Jen.
You dont have to explain yourself to a bunch of (vile) internet strangers.
But you do need to rationalize all this to yourself.
You have already had a lot of good advice about coping as a single parent. And to stay away from Jen, as this will only serve to confuse your kids.
Your wife left because she needed "a break" - You dont go move in your ex from more than 15 years ago (I assume) within a month of your wife leaving. This is foolish. You need to be a family with your kids. Not some other woman that swoops in.
It is very rare for a woman to abandon her children. Men seem to be able to do this without looking back at a mouth-dropping rate. But that is by the by.
You will find that many posters here are so keen to not apply double standard, they lose their usually no-nonsense attitude. This situation cannot be turned on its head, unless you both work full time and are as financially stable as the other.
But I doubt that, because it would mean procedures were in place, such as cleaner and after school nanny, and your life would not be as topsy turvy as it is now. But this is just me assuming. And admittedly, I assume a heck of a lot. 
Does your wife work?
Has she been a stay at home mum?
You say you have a well paid job that has allowed you to have a large family.
But I dont get a sense from your posts that you know your children very well, nor that you have been very hands on. I dont get a sense of you wanting what is best for them (your concern is Jen and coping, and bringing in Jen to help you cope).
When a man leaves a mum and their children behind, the problem is usually (aside from the emotional) financial. The working dad has the salary. Mum is left with the kids and does not know how to feed her kids. You are in the position that money is not a problem (as you work, you have your home) the problem is coping with your kids. Structure, boundaries, and a nanny are good suggestions.
This should be your focus, along with getting legal advice, try to find your wife and talk to her. Find out why she left and see if you can repair your marriage.
No Jen!
Good luck!