Explorer...yes, I have used Original Kit Bag many, many times with a full-backpack on. My new backpack, a Seek Outside Revolution 6,300, works beautifully with it but so did my 8oz Zpacks Blast. I always recommend putting the chest pack on first then the backpack over it. Then, you'll dock the chest pack to the backpack's shoulder strap, as they always have a daisy chain loop or two by which to connect. Then all the weight is transferred to your backpack.
I went out yesterday with my SO backpack and HPGOKB. I never enter the woods without it.
To answer your question about the straps...no, I've never had any problems with that at all. No crossing or combining of straps between the two. In that regard, Seek Outside sells strapkeepers. They're awesome to help organize straps. As you know, with any backpack, there are straps dangling all over the place. You can tuck them in or you can get strapkeepers which I think is the best solution. You roll up the excess strap dangling down and attach a strapkeeper which keeps it rolled up and out of the way. It's not permanent so when you need to use the strap, you simply unstrap the keeper which connects by using velcro then unfurl the strap. I bought two sets of 6 strapkeepers for a grand total of $5.80. No shipping cost either. Try them, I think you like it.
https://seekoutside.com/backpack-strapkeepers-set-of-6/
No but it will fit beautifully attached to my Seek Outside backpack! As Norse mentioned, you can get a breakdown 45-70. For a mere $3,277+, you can have a Wild West Guns Co-Pilot. Talk about a sweet lever action!
https://www.wildwestguns.com/custom-guns/