Hi Flute Fanatic,
I was a flautist who converted to the oboe, initially I did both but have left the flute behind now as I really want to focus on oboe.
I found that playing both did not ruin my tone on the flute, but as I was just learning, and teacher wasn't an oboe specialist I don't think the tone on my oboe - or my embouchure - was particularly good. But other people do seem to have managed.
Similarly with fingering - when I was regularly playing both instruments I didn't seem to have a problem with the different fingering. On the odd occasion that I have picked up my flute recently I have found it a bit confusing (especially F/ F#) but I think that would go away with practise if I really wanted to get my flute playing back.
I found that that splitting practice time between 2 instruments meant that I had less time for each so progress was slower on each one. Other people seem to manage multiple instruments, I have come to the conclusion that it just doesn't work for me and I find it much more sustainable to stick to one!
Generally the oboe is less forgiving of not being played regularly than the flute. Embouchure and stamina falls away very quickly.
I would also say, that it is essential that you have some lessons from an oboe specialist who can help you find a suitable reed, adjust your reeds and get your embouchure sorted out. Reed are very tricky and very personal - it is not really possible to just buy some from a shop and see how you do. You do need someone to help you! Bearing in mind that you say that you hate the sound of an oboe up to about grade 7 - you will struggle to get anywhere close without a teacher (and probably give up) and with a teacher you will probably be making a nice sound well before Grade 7 level. I had a general woodwind teacher for a while, quickly plateaued and then gave up. On restarting I have gone to a specialist teacher and the difference is ENORMOUS. But I have had to spend a lot of time getting rid of bad habits and there are vast holes in my knowledge which are slowly getting filled (especially about reeds). I think it was a mistake not to start with a specialist, and would have been even worse without a teacher at all.
When I started I rented a student model for 6 months to see whether I would like it or not. This was a much cheaper option and you may find that you can then fund some lessons - or at least send it back after 6 months if you don't take to it. I also alternated flute and oboe lessons on different weeks - so was still having (and paying for) only one lesson a week.
As you said, an oboe is a large investment of money and trying to do it alone is just setting yourself up to fail. A few lessons to get you on your way will really help you get the most out of it and you'll be far more likely to get that lovely oboe sound that you want! It is a lovely instrument to play so worth the investment of time and money! Good luck with your decision.
Hopefully this will be helpful.