Well done you for keeping at it - can be tough at the best of times without dealing with c section and tongue tie. Has anything been done for the tongue tie by the way? (Sometimes if baby cannot latch on properly they may feed more frequently)
Newborns have such tiny stomachs and breast milk is so easily digested that feeds can be very frequent at first. From hearing others experiences every 2 hours is not uncommon and when they are having a growth spurt can easily be every hour. I wouldnt say you are over feeding her at all feeding that often.
What you describe can be down to too much foremilk and not enough hindmilk but that isnt necessarily down to feeding frequently. I think you would know if she had this problem - crying after feeds, squirming and wriggling a lot, perhaps green nappies. In that case the advice is usually to feed from one side over the course of two or three feeds so they are getting to the hind milk. From what you say I dont think this is an issue.
If you watch a little baby, when they are hungry they do tend to start mouthing, sucking hands etc - crying is often the last sign, so by feeding them when you notice this you are doing a fab job. It is true - breastfed babies tend to regulate their own intake of milk so as long as she is latching on happily (and who can actually force a bf baby to drink?) then she will sort her own needs out. I know how exhausting this can be but it is worth it in the end. Take it as an excuse - if you can - to put your feet up and watch tv!
As for feeding every 20 minutes that is normal too and varies from baby to baby and feed to feed. Both my babies only ever fed for 10 minutes at a time - friends babies may feed for an hour or more. They are all different and it sounds like she is doing great.
I wouldnt try and stretch out feeds at this early stage. Babies and boobs are really clever - they work out together how much milk you need to feed them. If baby feeds, you make more milk to match her needs. That is what growth spurts and lots of feeding are all about - stimulaing your supply. If you miss feeds, not only do you have a grumpy baby but you may not make as much milk as your body thinks you do not need it. This is especially important in the first few months or so as your body gets used to working out how much it needs.
You sound like you are doing a great job!