i've had these kind of questions before too!
generally worrying too much about it is bad. i found that prayer is a good weapon against worries.
the enemy would rather we worry too much about fasting than spend the time reading the Bible and coming close to God through prayer and repentance.
i'll give you example of my 'silly' questions:
'is honey ok during the fast?'
answer: 'no, i can't believe you even asked that, i can't even give up milk and you are worried about honey!'
so i made my friend feel bad because i asked this. i didn't realise she was having trouble with fasting.
in fact, honey is allowed during fasting, even though strict vegans don't eat it.
another 'what do you do if you have a loose corner of your finger nail that needs biting off?
do you bite it before Holy Communion and risk invalidating your fast if you swallow it,
or do you bite it off after Holy Communion and so risk bleeding (which should be avoided)?'
answer: 'don't bite your finger nails before or after Holy Communion, it is not a nice habit!'
(it will not invalidate your fast, but really you should try to avoid bad habits!)
as for food stuck in teeth, the way i see it is that fasting is avoiding putting something in your mouth (food, cigarettes, gum).
so if something is already there, that is fine (although bad for your teeth).
brushing in the evening should help here; or you can use a tooth stick if you can get one from africa.
i love tooth sticks! if i could get them in uk, i would use them every day.
as for fasting on feast days, this varies widely between churches.
for us (as far as i can work out),
only the nativity feast (Christmas) and epiphany (theophany) are allowed to break the fast, but don't worry about our rules as coptic fasting is famously strict.
about how strictly you fast, this is between you and your priest.
if you break every fast in the first few days, maybe you are trying to do too much and you need to just avoid meat but eat fish, oil, eggs etc.
then you can slowly build up to 'full' fasting. but it depends on your priest.
ask God for some spiritual friends in your church who love to fast and who are welcoming and kind,
and do what they do, unless your priest has told you to fast less.
may God guide you, and remember, when you feel the need to worry, keep praying until it goes!
(i have this problem quite often and praying is the key)