I have feelings for a shia (and I want to marry him) and I want it to be as halal as possible. My family took out istikhara. They did from 2 different places

One said our marriage will be successful for only 2 months and later on issues will arise and they won't settle, instead they'll grow worse, so much so that it will cause our marriage to end. Where as the second istikhara claims that, the guy is good and ons his end the things are pretty clear but on my end things are not so clear, and there are problems on my end

These are two istikhara's taken out by our sunni maulana’s.

However I also made an istikhara, and that was done my Maulana. He made it with tasbeeh,
He told me to make an intention and he'll carry out the istikhara, and he did and told me that it's ok, good ( he made it via tasbeeh )

How is it possible that one istikhara gives a positive result and the other 2 claim this? If it's good, it has to be good all over, how come it's so dismantled?

Istekhara should only be taken when there is utter confusion on what action should be taken. In your case, it was so obvious for you that the spouse you had selected for yourself matched the right criteria, therefore, there was no need of istekhara in the first place. If your parents did that without your deep consent even if you outwardly proclaimed otherwise, the istekhara is null and void because you weren't for it due to being sure about your own decision.
On the other hand, scholars say that whenever two istekharas are taken, there should be a different intention for both of them regarding the matter for which it is undertaken. In your case, your parents kept the same intention for both the istekharas so there is a problem to that too.
Thirdly, the person who takes out the istekhara also matters. The more pious and God fearing the person is with the right beliefs and mindset, the more the answer is appropriate.
There are so many deeper meanings of the versus of the Holy Quran that only the awliya of Allah (chosen ones) can understand it so if the istekhara said the marriage would last good for two months, there is simply no proof for the validity and i believe it is superstitious to go for that.
If the shia Mawlana took out the istekhara with your consent only and not your parents, then it is the right one. It is left on you though to convince your parents about this.
Another option is to communicate to your parents and make them understand that it is your own life after all and you yourself have to make the decision, so you have decided to get married to the man of your choice by laying full trust in Allah the Almighty who loves it when decisions are made and trust is kept upon Him, and the outcome will follow with His mercy and blessings InshAllah.