Name : Pranav Bhalla
Your queries –
1) If astrology is science which has it’s deep roots in various shastras, why is it that different astrologers give totally different analysis and predictions on the same two kundlis of girl and boy for match-making ?

Astrology is based on the movements of Planets, sign etc. From 0-30 degree we call it house, means 360 degrees divided by 30 will give us 12 houses. Every house has its own signification, and sign has own signification + over and above planets and their lord ship their signification, aspect on planets will guide us impact on signification on results by house, sign, planets etc. Now if we work out on the basis of permutation and combination will sum up to a big figure.
Now, every person who is reading this chart will work out and give his or her opinion, and it is difficult for a reader of horoscope to analyze all the significations for the natal or natal is also not ready to understand so many significations. Natal chart reader will give main events (according to his/her point of view) to the natal. Normally you will find that all the events told by many astrologer will happen but you may not be focused on those events.
Natal is thinking that these analysis are different. But many times you will see that all the analysis are close, but way of presentation is different. I had seen many times, natal will say “Yeh toh humko kai logon ne bataya hai, aap nai baat bataiye”.
Say for example horoscope of Chetan sharma – will reflect income from sports, but as well from voice, now houses involved in this horoscope will misguide astrologer – house of efforts, courage, competitiveness will also reflect legal matters, health related issues, etc. now any one can say may be from Legal profession etc.
in my opinion better to ask few points from natal so that we can be on the right track, there is no harm in asking from natal. In old days few professions were there, now we have plenty of them, so better to discuss with natal and planets+sign+house etc. and then give own opinion.
Same thing is at the time match making – they are just going for point tally. In fact if genuinely horoscope has to be tallied, then it will not tally so easily. Parents/natal are/is not interested in such type of match making.
They are analyzing pros and cons according to their point of view, only difference will come in their opinion. Regarding tally of points (Gun Milan) majority of persons are seeking help of software, so no issues, almost same result.
Query 2) Can marrying a manglik boy ever become life threatening or fatal for a girl who marries him?

What is Mangalik? – mangalik means Placement of Mars in Ascendant (Lagna – self), second house – house of Kutumb, fourth house – house of family, happiness etc. seventh house – house of spouse, eighth house – house of Mangalya, and twelfth house – house of bedroom pleasure.
According to shstra we must work out from Lagna, Moon and Venus. But now a days due to  marriage problems normally Mars from lagna is checked.
Now, point is Mars is known as Malefic planet. According to Shastra – balancing of planet will solve the purpose. Means any other strong malefic planet is placed in the same house or is balancing Mars will solve the purpose.
Otherwise Mars will give impact according to the house of the natal.
So my opinion is different for this query.

Query 3) Isn’t life and death dependent on a person’s own kundli and karma?

Death of the natal is based on his or her own planets, but will reflect in horoscope all the family members such as son/daughter/spouse and sibling etc. even in the horoscope of parents too.

I think, its a very deep subject, and only within such lines it is difficult to explain.