Given what you have described, the refusal notice is not altogether unreasonable. Your wife is settled in the UK with ILR and you have a child born there who acquired British citizenship from her mother. Your employment situation is not the best and you haven't been paid regularly. I assume that you and your wife cannot meet the financial hurdles in Appendix FM to bring you in as a spouse.
From their viewpoint it paints the perfect picture: you have a secondary agenda of family reunion and want to live in the UK as a spouse, but cannot qualify for that type of visa. So to avoid the qualifying hurdles in Appendix FM, you will enter the UK as a visitor and then go underground. According to the pattern, this would culminate in a derivative rights claim (they don't like those and try to turn them off early on).
Perhaps that's your plan, perhaps not. But for sure that's what they were thinking. Everybody in the world says it's not their plan to settle in the UK obviously, and to counteract it they will use any reasons they can however captious. That's why it's doubtful that your proposal #1 will make a lot of difference...
I have asked my wife to send our marriage certificate and our
daughter's birth certificate. I will also add her flight tickets, a
few wedding pictures and Skype call logs as proof of relationship.
They will not care about flight tickets and photos and Skype logs can be too easily forged. And at the end of the day, the best this evidence can do is support your claim to being married to your sponsor. But it does nothing at all to mitigate the refusal grounds...
(a) will leave the UK at the end of their visit; and
From the Visitor Rules
For under GBP 100, you could commission a DNA test to prove your daughter's relationship. It's not required and would be very extreme for a visitor application, but again proving direct lineage would not clear you of 4.2 (a) (cited above).
Your salary situation has been stable for 4 months, is it long enough to offset what they said about '...unless the circumstances of your application change'? How long before that was it unstable? ECO's think in terms of 6 months to a year on things like that.
And finally, against that backdrop, changing your proposed visit from 28 days to 18 days will not be much help. If you are planning to abscond (based on their viewpoint), you need a visa lasting for only 1 minute, just long enough to clear the primary control point at the airport. The so-called "best practices" advice is to set up your visit to be consistent with what you want to do and not what you think will make them happy. Visit duration should be consistent with lifestyle, employment circumstances, planned activities, and so on. The Standard Visitor Visa runs for 6 months anyway.
Your questions...
Please, do i stand a chance of getting the visa?
We don't take bets one way or the other and we cannot assess your chances without seeing lots of stuff and learning your personal circumstances. Nobody can.
Is there anything i can add to increase my chances?
Minimally you can wait until your income/employment has been stabilised for a convincing amount of time.
Secondarily your wife and child can visit you in your home country or in a 3rd country, they like stuff like that but by itself it will not carry the application.
Also, can i use priority for the visa processing?
If priority services are available at your VFS/VAC then generally yes but it depends upon the country and VFS within the country. At least you can PAY for priority. Your case is "complex" and will require the processing associated with complex cases. That obviates the possibility of anything happening quickly. Worse, they will keep the extra fee you paid to get prioritised.
