Will he be allowed to walk in front of and through rows?
If he leaves and a gap is created in his place, how should other praying muqtadis close that gap?
If there is no chance of him coming back in time for remaining prayer, should he still leave?
Please also tell what happens in a jamaat of only two persons, where either the imam or muqtadi breaks wudu?
|Bismillahir Rahmaanir Raheem|
Al Jawaab Billahit-Tawfeeq
If the follower (Muqtadi) breaks Wudhu in congregation (Jamaat) and is in middle rows, he should immediately go for Wudhu. 
Yes, he(the Muqtadi) will be allowed to walk in front of and through the rows(Sufoof) providing that whilst walking past them(the followers) he doesn’t move their chests from the direction of the Qiblah and invalidate their Salah
The followers (Muqtadis) can close the gap by moving along the row (saff) or by moving from the row (saff) behind to fill the gap, if one moves from 2 rows (saffs) behind it will break the salah as this is amal e katheer. 
Because these masa’il are complicated, and because of a lack of knowledge – the possibility of mistakes is inevitable. It is therefore better not to continue the salaat. Instead, one should break the salaat by making salaam and repeat the entire salaat.” 
A detailed chapter upon this subject can be found in Heavenly Ornaments (which is an English Translation of Bahishti Zewar), Chapter of Salaat, The Experiencing of Impurity in Salaat page 200-203, Zam Zam Publishers Print.
The book can be purchased from the following link:
I advise you to read this.
” once he experiences any hadath, he should not delay in stopping any rukn without any valid excuse. Instead he should immediately go to make his wudu. However, if there is a valid excuse, there is no harm in delaying. For example, there are many rows (saffs) and he is standing in the first saff and it is difficult to break through the rows (saffs).”
It states in fatawa mahmoodiyah:
“In every row (saff) one should pass between 2 muqtadis(followers), this is permissible. However if it is difficult to leave, he should sit in his place and not take part in the salah”
It further states:
“If he can pass by the muqtadis(followers) in such a manner, that their chests do not move from the direction of the Qiblah and invalidate their Salah he should leave, otherwise he should remain seated in his place”.
In Ahsan AL Fatawa it states:
نماز كي اصلاح كےلئے سامنے سے گزرنا جائز ہے ، لہٰذا جاتے وقت سامنے سے گزرجائے اور واپسى تكـ اگر وه جگہ خالى ہے تو سلمنے سے گزركر اس جگہ كو پُر كرے، بلكہ سامنے سے جانے كي جگہ نہ ہو تو صف كو چير كر بھي جاسكتا ہے،
“For the rectification of Salah, it is permissible to pass in front of the muqtadis, therefore when leaving one should pass in front, if the space is still empty upon return, then on should pass in front and fill the space, and if there isn’t space to pass in front then one could break through the saffs .”
It also states:
” It is permissible(to break through the saffs), though instead of breaking through the rows (saffs) to pass in front is easier and better.”
It states in Bahishti Zewar, Chapter of Salaat, The Experiencing of Impurity in Salaat, Page 202,
“5. If a muqtadi experiences hadath, he should also immediately go to repeat his wudu. On completing his wudu, he should join the jama’at if it is still in progress. If not, he should complete his salaat on his own. If the jama’at is still in progress, he should go and stand in his original place. However, if there is nothing between him and the imam which can prevent him from following the imam, then it will also be permissible for him to stand at the place of wudu and follow the imam from there. If the jama’at is over, he has the choice of going to his original place and offering his salaat, or of offering it at the place of wudu. This latter option is preferable. “
ثم رأيت في مفسدات الصلاة من الحلية عن الذخيرة إن كان في الصف الثاني فرأى فرجة في الأول فمشى إليها لم تفسد صلاته لأنه مأمور بالمراصة
من سبق الحدث في الصلوة انصرف فان كان اماما استخلف و توضا و بنى و هو قول الشافعى الخ و الاستيناف افضل تحرزا عن شبهة الخلاف
(print هداية ص128 ج 1,باب الحدث فى الصلوة, المصباح )
Fatawa Mahmoodiya (Page 572, Volume 6, Jamiah Farooqiyah Karachi Print)
(استيناف بہرحال افضل ہے كيوں كہ جوازِ بناء كےلئے تيره شرطيں ہيں جن كي حفاظت ہر شخص سےدشوار ہے)
” to do Istinaaf (pray the salah from the beginning again) is better, as for the permisibilty of Bina (continuing from where one left off) has 13 conditions which are difficult for everybody to follow).
And Allah knows best