Marriage

Bismihi ta'ala

I would strongly recommend you look into the functionality of mahr, as unfortunately many Muslimshave completely misunderstood what mahr/sidaq is for. 

Mahr is nothing but a gesture and tokan of the groom's truthfulness in his proposal for marriage, and once the nikah is done, you both live amicably, with utmost respect, love, commitment and mercy. 

Should you wish to spend some money on her, or take her to Hajj, or buy jewellery for her, or property in her name, or anything else, that is out of your good will, and can be used by both of you as an investment for yor future as a family. 

It does not need to be under the title of "mahr", and none of these things work as a bond or a security for the continuation of the marriage or good spousal treatment. 

Islam teaches us to keep distant from developing a materialistic mentality, and marriage should never be about money or wealth. 

And Allah knows best

Bismihi ta'ala

A Sayyid male/female is like every other Muslim, nad can marry a non-Sayyid/non-Sayyidah, Shi'ah or Sunni, with no difference. The one condition is she has assurance that she will not leave the Ahlul Bayt School of Thought, or negatively influenced or forced to become Sunni. 

As for parents disapproval, there must be correct understanding between families, and one must always try to get the consent and blessings of parents. 

And Allah knows best. 

Bismihi ta'ala

Yes, if it is crucial information and would affect the success of the marriage, and you are asked for your input, you must be honest. 

This situation is among the cases where gheebah (backbiting) is permissible, or maybe could even be obligatory. 
Of course, you must be careful that it does not somehow backlash onto you as well. 

And Allah knows best

Bismihi ta'ala

Firstly, we should refrain from using the word dowry, because in English it is a payment given by the bride to her husband. 

Whereas, in Islam, it is from the groom to the bride, and not a payment, but a token and sign of loyalty and truthfulness, which is why in the Quran and Sunnah it is called "Sidāq". 

We also call it Mahr, and in English, we can call it bridal-gift or nuptial-gift. 

Once the marriage contract (nikah / 'aqd shar'i) is done, it becomes the husband's duty to fulfill the contract and what he comitted to. This obligation depends on the agreement the bride and groom had. Was there a clause in the contract, like dividing the mahr into "muqaddam" or immediate, and "mu`akhar", which is the deferred amount. 

Is the husband capable of giving it immediately. Has the wife demanded it? Can the husband give it in installments. These are different scenarios that could occur, so it would depend on the circumstance. 

Some cultures also have it to be given on the day of the nikah as well. 

So, from a shar'i perspective, it becomes wajib upon the husband to give the mahr that he agreed on, whenever it is demanded by the wife, whenever he is able to, and according to whatever agreement they had. 

And Allah knows best